1. Grab an AI unit to sit on the chair, name him G1
2. Get an empty chair and name it chair1
3. Place the following commands in your init.sqf (Mine is in my Mission1.sqf)
//------------------------------------------------------------------
//Attaches G1 to chair1
G1 attachTo [chair1,[0,-0.15,-0.5]]
//Rotates G1 in relative direction to chair1
G1 setDir 180
//Plays the sitting animation
G1 switchMove "InBaseMoves_SittingRifle1"
//------------------------------------------------------------------
Run the mission as another unit watching G1.