Custom BeagleBone Black hardware manufacturing?

Hi,

I'm just about to start with embedded system and linux and BBB seem pretty good for this, it's open hardware that I can customize in the feature, but I wonder how I can get that manufactured? Any service that you can upload part/ pcb and they build the board for you? Preferably something that accept low quantity for testing.

Also is it a good idea to customize bab rather than design from scratch for save cost/time?