Buffs do nothing in this game. Giving a unit the Possession buff simply means, a neat little graphic will appear over their head and a cute little box will pop up at the bottom of the screen, but buffs are almost* totally worthless. Like SilverHawk said, changing the autocasting might work. However, there is a trigger solution...
Events: Unit within Range of (Possessor)
Conditions: Owner of (Triggering unit) Not Equal to (Owner of (Possessor))
Actions: Order (Possessor) to use ability Possession on (Triggering unit)
Might work, might not. I haven't tried it, myself, and of course Possessor would have to be a Unit-type, but that would be the general idea. Otherwise, just base it off an Autocast.
*They can be used in trigger references, as a condition, but unless you're going into some fairly detailed triggering, they serve no physical purpose.