At first of all they could be usefull only for strat siege and maybe some open field battles.
You need to have new kind of arrows with 1 cut dmg. Additional dmg from fire will be only to siege equipment. The only exception to this should be siege towers (they were made of wood, but covered with fresh animal skins, that are very hard to burn).
How can I see using fire arrows?
Next thing you have to do is build a "Source of fire" (to make it you should have a cs and about 5 or 10 cm). It can looks like a barrel with flames in it. To fire up an arrow you have to go to source of fire and press F for about 1 sec. Now you have a burning arrow ready to shoot.
One more thing: if it will be implemented into game, non burning arrows should do no dmg to siege stuff.
If not dmg from arrows and bolts should be reduced to about half of present one against siege stuff.
Archers can't be too OP XD
Happy whining