I think you have only this possibility of having a crouching unit:
_unit switchmove "Crouch"
or
_unit playmove "Crouch"
It works with OFP v1.00!
If it doesn't work with your OFP version, explore the config.bin file at C:\...\OFP\BIN, by using NotePad(win2000) or Wordpad. Seek for known moves like "StandToSitDown" and copy all the moves you may find...
Wish you luck!