Put down a chair, name it to something (like chair_1).
Then either use a script or trigger to set the anim and the soldier on the chair with:
unitname switchMove "miles_c0briefing_odpovedel_loop"; unitname setPos (position chairname)
unitname of course being the name of the soldier and
chairname the name of the chair.
And the code has to be run from a trigger or a script, using the unit's init field doesn't work in this case..
You can find the available animations from
Arma2 moves.
I think the "old"
ArmA moves work too
EDIT:
Note that the soldier will stay in that anim forever, even if it dies.
Dunno if it's because it's a looping anim (judging from the name) so some other anim might be better