Ghouls make other ghouls in both the old fashioned way (by breeding amongst themselves) and by trading ghoul babies for human babies. For some reason humans who spend a lot of time in the company of ghouls usually become ghouls themselves. This is especially true for human babies raised by ghouls.
Ghoul babies raised by human beings will eventually become ghouls as well. It just takes them longer than if they actually raised by ghouls.