You can use colony ships or troop transports to ferry colonists from one planet to the next. Once they reach their destination, the colonists are automatically added to the population of the colony. Both ship-types, however, need at least 1 million people on board or they won't leave the orbit.

IIRC, if one tinkers with names, one can get a home system with a normal PQ 10 homeworld, but with also a PQ15 (IIRC) other world in that system.  If that planet is only one square away, one can transfer all the population you want to the other planet in one turn even with an engine-less colonizer.

Even if no tinkering done, one usually has a PQ4 planet in the home system to do the same.