Thank you again for the link! It took some time to have some time for this, but it is pretty straight forward. Find the relevant dn (in my case bareos-enabled):
you@ucs:~# udm settings/extended_attribute list --filter cn=bareos-enabled
and modify it:
you@ucs:~# udm settings/extended_attribute modify \
--dn "cn=bareos-enabled,cn=bareos,cn=custom attributes,cn=univention,dc=domain,dc=local" \
--append module="computers/linux" \
--append module="computers/ubuntu"
Bernd