Well, in modern equipment, how many devices use GALs?
I think you can... Make a library with your custom parts. Put it in the list of libraries first. KiCAD searches in the order of the list and uses the first match.Anyway, you can make your own parts and you can take advantage of doing that to your hearts desire. It is just that you can't use the name 7400. At least I've not found a way.
Also remember feedthrus are space hogs so take advantage of IC pins for these whenever possible. ( I mean expensive in space ).