There is a "sit" script available in the FPSC scripts forum.
It worked for the EA version but not in this one.It makes the character kind of sink into the floor a little.
But if you put the character behind a table or desk it will appear like they are sitting.
You can run your game and walk up to the character and tell it to sit down or you will shoot it.
Of course,the character will not hear you screaming into your monitor or sit down but you do get the satisfaction of blowing the character away.
The only other option I can think of other than using the "sit" script like I mentioned above is to rig the character like has been previously stated.