Now, I don't mind my speialists being restricted a bit ... that goes with the territory with themed warbands .... but I do need to come up with something for thieves and thugs ... I was thinking maybe some monks? But then the brown robes would spoil the painting scheme if not the theme. Might Just have to give them a warhound so they can afford to start without thieves and thugs and still have an apprentice.
I like the idea of using lay brothers as thieves and thugs. Since they cannot be ordained they are relegated to the menial roles of armsmen (thugs) and foragers (thieves). Keep your color scheme, but replace the white robes with grey or light grey robes.
A simple archer conversion is to replace a hand weapon with a sling. You can sculpt one using a pin vice, wire and greenstuff or you can simply use greenstuff to sculpt a sling hanging from a Sister's belt. Alternatively you can do a hand swap with a singer model. There are several slinger hands on Gripping Beast's Dark Age Warriors sprue. The sling hand will be a bit small for a GW model, but you can use greenstuff to make the hand larger. If you can't get your hands on the sling bits I can mail you some. I have plenty of extras.

Satchels for your slinger's stones would be a nice addition. If you don't have proper satchel bits you can sculpt satchels. It's fairly simple. I added GS satchel with shoulder straps to all of my SAGA Irish levy. Below are some WIP pics.


Pathfinder Miniatures' Nexian Bounty Hunter can serve as either a crossbowman or marksman.

A treasure hunter conversion is easy. Start with a Sister model that isn't armed with a 2-handed weapon and add a backpack and bunch of stuff like satchels, pouches and coiled rope.
Add a staff to a slinger Sister and you got yourself a Huntsman.
An easy barbarian conversion is to start with a 40K Sister Repentia and replace the eviscerator blade and hilt with an enormous sword blade. And sure, they'll get awfully cold running around in their leather girdle and undies, but that's just another part of their penitence.
