I'm not sure if you can, but you could try to let some russians join a NATO team, maybe that works, but i don't think so! :-\
try:
[RSoldier1, RSoldier2, RSoldier3] join NATOLeader
replace the 3 RSoldier* with the names of the soldiers you wanna have to switch sides and replace NATOLeader with the name of the group leader of your NATO group.
Place it in a trigger or something like that, try it, i really don't have any experience with the switching of sides in OFP.
If you don't have a squad your soldiers can join, well, i think there could be some workarounds, but not sure if they would help you out.
Tell me what exactly you've planed. Maybe i can give you more hints.
Bye