the UVMM (the web-based management component for virtual machines) doesn't support USB pass-through at the moment. There don't seem to be any concrete plans for adding such support either (see e.g. this bug report in German: 30440).
However, there are reports by users who have succeeded in implementing pass-through by editing the corresponding configuration files (via the "virsh" tool). See e.g. this post (unfortunately it's in German, too). On that layer you're not doing anything that's specific to Univention, though; this is basic KVM/qemu management with libvirt. Therefore you can also Google for a general howto about USB pass-through with libvirt.