Yep. You give them a script in their "OnSpawn" event to make them sit and then in the conversation on the very first line in the "ActionsTaken" tab you basically give them the same script. This just requires that they have at least a one line conversation. Example script:
void main()
{
string sChair = GetLocalString(OBJECT_SELF, "MY_CHAIR");
object oChair = GetObjectByTag(sChair);
if (GetIsObjectValid(oChair))
{
ClearAllActions();
ActionSit(oChair);
}
}