good news and bad news. Bad news first:
You should definitely avoid adding Debian repositories (or repositories made for Debian/Ubuntu instead of Univention). Univention's versions are far enough away from standard Debian that you'll run into problems eventually, e.g. during upgrade time. Don't do it.
The good news is that the Univention repositories include all of Debian's software, including editors such as joe. They're just not part of the officially supported set of packages. Univention calls this distinction "maintained" vs. "unmaintained". You can simply activate the "unmaintained" repository via a UCR variable and install joe from it. Easy as pie. See section 5.5 of the admin manual for instructions.
 You can safely use Debian source packages and compile your own .debs on a Univention system. This works better if the source package comes from a version of Ubuntu/Debian close to the current Univention release, e.g. Debian 7 Wheezy or Ubuntu 12.04 for Univention 4.x.