I think you can move a monster using addto_object_property for its X and Z coordinates, but that may not be the best way.
But a better way is to create an invisible generic object and tell it to go where you want the monster to go. Then constantly use set_relative_position to pin the monster to the generic object. Once the monster is engaged in combat destroy the generic object and stop pinning the monster to it.
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).