Create a flag and in it's initialisation field use:
this setflagtexture "usa_VLajka.pac"
This will put a flag on the flagpole (use "Rus_vlajka.pac" for a Russian flag) Then give this flag a name like ourflag. Then you need to give the flag to the unit. This is done using the setflagowner command. For example:
ourflag setFlagOwner FlagCarryingDude
This will put the flag (ourflag) on the back the unit specified (FlagCarryingDude). This can be put in triggers or in initilisation fields.