I am playing on an insane map, with lots of planets.  I attacked the Altarian Resistance, destroyed a couple of ships, blew up a space station.  My transports were out of place, because last war ended too quickly by that race surrendering remaining planets to another race.  The Altarians also surrendered to another race.  GRRR.  But I went into the trade window, started at bottom of planet list, and traded for all the planets the Altarians just surrendered for nothing.  I gave no money, resources or planets.  I though huh, that was little weird.  Well I did it to several other races in game.  I found that if I stacked 15 planets on trade list, it went from "You must be joking" to "Though our ideology is different we accept the trade"   I just conquered, easily half of the map that way.

I wonder if the "cost" of the trade hits some amount and it rolls over into "acceptable" range.