How to make packages from f-droid to be treated as from a "known source" (if I have root)? I usually get packages from http://f-droid.org/ 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.
Almost every F-Droid app is signed with a different key, though all keys are in the same key store. In order for the switch to be bypassed, F-Droid client would have to be installed as a system app and made to work safely as such. Some code has been supplied for this to work, but it hasn't been integrated yet.