I'm not sure it is possible :s.. Even if they have the same base, there is a
lot of change in the background so you may see more breakage than anything
else (and also a lot of unavailable packages when updating).
Your Debian is Free just after you installed it, the only solution to become
non-free is to enable the Contrib and non-free servers (since they use their
own meaning of Free Software to select which package will be in which
repository, that is not totally compatible with the FSF meaning of Free
Software).
The Debian Kernel is Free and all the packages in main are also as free. So
just don't enable the rest of the repositories and (if I'm not wrong) you are
good to go :D
