A custom is an item that is significantly altered or scratch-build (not just slightly altered, say where only the bumpers of a model car are painted).  Those can be added to hobbyDB with their Production Status marked as Custom.  Their Brand should be the Customizer or left blank if not known. 

Greet Your Liberators by Suckadelic

If they are based on an existing model and are still resemble that model they can be varianted to it, like this custom based on Hot Wheels Kool Kombi  -

Ghostbusters Kool Kombi by Brew Customs