How to make packages from f-droid to be treated as from a "known source" (if I have root)?

I usually get packages from repository through their package browser (because then I have a guarantee that I use free (= libre) software); and they are signed with their key. I use CyanogenMod 10; and I have root access. I don't like allowing packages from "unknown sources" for this. Can I set up the system so that f-droid counts as a known source, and packages from all other unknown sources are not allowed to be installed by default as before. Perhaps, something can be patches in the CyanogenMod distro to add f-droid's key as a key for a "known source" of packages.