It all depends on exactly what you are wanting to do. The easiest way would be to make a second NPC to manage the one you are trying to set to respawn. Then just have them do
Code:
quest::depop(ncp_id);
quest::spawn2(npc_id, 0, 0, x, y, z, h);
And have that second NPC keep a timer of when do "repop" him. Otherwise, you could just have the NPC itself do:
On itself and then set the respawn timer on him to like 1 second.
There are plenty of other ways to accomplish what you are wanting to do, but it depends on how complex you want to be :P