Thought I would share this since I remember trying to do this in the past and couldn't figure it out.  There's a way to include the non-free repo in your plan.  In my case, I wanted to include snmp-mibs-downloader.

What you do is you edit the Makefile, and on a line somewhere above the include line, you put:

NONFREE = Y

There are a number of other variables you can use here.  You can see them if you run: 'make help'