It'd certainly be possible, but you would have to add in some sort of way to make them do it.
The easiest way to give them mercenary troops (or just different troops) would be to edit party_templates.txt to include mercenaries. The templates are like this:
pt_kingdom_1_reinforcements_a kingdom_1_reinforcements_a 0 0 1 0 35 2 6 0 34 4 7 0 -1 -1 -1 -1
That's kingdom 1's reinforcements_a.
35 is the troop type, followed by
2 6 which is the amount of that troop (a range). It then has a
0 for whether or not they are a prisoner (1 = prisoner, 0 = regular member), and repeats for the second troop type. The four -1s are the other potential slots for members. To add another troop type you would change one of the
-1's to a troop number and then add in two numbers for the amount and a 0 so they aren't prisoners. You can have a max of 6 troop types in a template.
To get the troop # you have to either open the unofficial troop editor and see the number there, or take the line # of the name in troops.txt and divide by 7 then subtract 3.
It gives castles/towns reinforcements_a 65% of the time, and reinforcements_b the remaining 35% of the time. Heroes get reinforcements_a 50% of the time, reinforcements_b 25% of the time, and reinforcements_c the remaining 25% of the time.
Kingdom 1 is Swadia, 2 is Vaegir, 3 is Khergit, 4 is Nord, 5 is Rhodok.
I'm not sure if this is savegame compatible or not, I've never actually edited party templates during a game. It would quickly become apparent if it wasn't, though.