Merry Christmas - I think that's going to be a fun project!
For fantasy gamers, some of the Frostgrave kits have some great bits that might come in handy - things like scimitars, crossbows, torsos/legs in something of a similar costume style, heads with turbans, and the like, not to mention some other great fantasy weapons, wizarding gear, and adventuring equipment.
These Frostgrave kits are built to a more "heroic" and exaggerated proportion - bigger hands and feet, that sort of thing - so the arms might not swap very well between kits, and the heads attach on a ball-and-socket attachment at the bottom of the neck, where WA's historical figures attach with a flat fit at the top of the neck, so the heads won't swap very well either, but with a little creative modeling you can make swaps work.
Otherwise, the real value with the Frostgrave stuff would be to use the Frostgrave bits on Frostrave figures to work alongside the Afghans as "hero" figures: chieftains, wizards, specialists, archers, and the like.
So, with that in mind, and the fact that Frostgrave kits are designed to mix-and-match bits between them:
- Frostgrave Crewmen - "Sinbad the Sailor" type costumes and turbaned heads, scimitars and curved daggers, and small round shields in a similar style to those in the Afghan kit, plus other fantasy weapons and gear (spears, bows, crossbows, bows and arrows, axes, etc.)
- Frostgrave Cultists - Costumes are light padded armor or tunics over very loose, baggy trousers and sleeves, plus some assorted evil-looking fantasy bits like bizarre and exotic-looking scimitars, krises, crossbows, bows and arrows, spears, and the like, good for adding something of the more exotically sinister to a fantasy war band of this sort. (Also includes some odd undead bits for undead conversions, for whatever that's worth.)
- Frostgrave Wizards I - Robed costumes with loose, baggy sleeves, heads in turbans, fezes, and other odd hats and such; a huge amount of wizarding gear such as wands, staves, spellbooks, talismans, and the like. Use the arms/hands holding spellbooks to build Mad Prophet Abdul Alhazred type characters.
- Frostgrave Soldiers I - The most interesting bits here would be additional bows and crossbows, plus the nice selection of non-weapon gear like arms holding lanterns, and backpacks, coils of rope, and the like. Also includes heads in leather caps and metal helms, including some in some exotic styles, if you want to add some armored characters.
- Frostgrave Wizards II and Frostgrave Soldiers II - the female counterparts to the other Wizard and Soldier kits, which include additional weapons, wizarding gear, and adventuring gear - all interchangeable with the male bits, and in different styles, for added variety; the Wizards II kit includes hands making magical signs for spells/curses, which is a nice touch. Aside from the nice variety of additional bits, the bodies and heads can be used to add female wizards and warriors to your army.
- Oathmark Human Infantry, etc. - mostly compatible with the Frostgrave kits, but best used as a source of spare generic bows, spears, and shields.
Numbers of figures: Note that Oathmark kits - made by the same company as Frostgrave kits and mostly compatible - include enough parts for 30 figures, and most Frostgrave kits include enough parts for 20 figures, but the Wizards kits and the Knights kit only includes enough bits for 10 figures - what the Wizards and knights lack in numbers, they make up for in variety, as the include tons of different weapon, head, and gear options for cutomizing some unique characters! The other Frostgrave kits include a smaller but still decent variety of weapons in different styles for a mix of melee and ranged soldiers, while the Oathmark kits don't have much variety, but do include enough bits to make 30 uniform, rank-and-file, generic spearmen, archers, or melee soldiers.
Spare Bits: Oathmark Human Infantry, Goblins, etc. made by the same company that makes the Frostgrave kits, the bits are generally compatible between the kits. Best used as a source of spare bits to supplement the Frostgrave stuff: a Frostgrave kit probably won't include enough generic shields and spears or bows and arrows to arm an entire platoon of archers or spearmen, but the Oathmark kits will! For your project, the (human-sized) Goblins set has some great orcy curved swords in it which might serve as extras scimitars if needed, as well as shields that might mix well with the Afghan figures, while the human set is a great source of extra spears and bows. A box of the Crewmen or Cultists combined with a box of the Oathmark Infantry should be enough to get you a good group or two of archers or spear-men, if you need lots of spearmen or ranged soldiers, and prefer archers to the default riflemen in the Afghan kit.
I've used the bare, muscular arms from other kits - Frostgrave Barbarians or Tribals, I think - for big, two-handed swords as the base to swap the sword blade with a scratch-built 2-handed scimitar blade made from some of the heavier styrene card from random samples of styrene materials made by Evergreen for railroad modelers - it's a similar hard plastic to the stuff the figures are made from, and glues together for a pretty solid bond with the usual cyanoacrylase Wacky Gloo, and is pretty easy stuff to work with for crafting and modeling. Combined with a Frostgrave Crewman body and a bald head with ear-rings that I got from, I believe, the Wizard kit, this guy served as the big, heavy weapon guy for the warband.
Other than that, it seems to me that the Wargames Atlantic WWI french kit includes sprues with scimitar-like machetes, if you're looking for something a little different for melee weapons for the WA Afghans; it looks like the upcoming Boxer Chinese set ought to include a good source of curved swords and other melee weapons that ought to swap well with the Afghans, if desired. WA's Dark Age Irish are a great alternative source of spears and shields, while WA's Persians are a great source of spears as well as some bows and arrows in a nice authentic (if ancient) middle-eastern style.