UCS supports creating additional ("extended") attributes for all object types.
There is documentation on creating extended attributes here: http://docs.software-univention.de/developer-reference-4.2.html#udm:ea
To see existing extended attributes run:
udm settings/extended_attribute list
Or graphically in the UMC:
Domain → LDAP directory
univention → custom attributes
Click an item to edit and see the same object as on the command line or:
Add → "Settings: Extended attribute" to add a new extended attribute.
You will need an LDAP schema. See the documentation on how to create and activate it. I recommend to go the long way of making a Debian package if you plan to maintain this for a long time.