Taken from Bioware forums:
1. Make a Chair placeable
2. Set the Chair to be useable
3. Put this script in the chair's onused hook:
void main()
{
object oPlayer = GetLastUsedBy ();
object oChair;
if (GetIsPC (oPlayer))
{
oChair = GetNearestObjectByTag ("Chair", oPlayer, 0);
if (GetIsObjectValid(oChair) && !GetIsObjectValid (GetSittingCreature (oChair)))
{
AssignCommand (oPlayer, ActionSit (oChair));
}
}
}


