Yeah you could do that in Gal Civ II I think or was it MoO2?
That would be really cool, being able to spread some pop from one of your massive urban sprawls to one of your destitute cities on the fringe.
On the flip side I think the AI would use this far more efficiently than the player could, and could possibly make the AI overpowered because it could build super cities even on the crappiest soil. Unless you could only add the pop once, i.e. when you build the city. So using the OPs original idea would work. Building 60 pop settlers instead of 30.