It's possible to add ClusterUserDefinedNetwork and namespaced UserDefinedNetwork as secondary network although they are already in use as default Pod network.

4.19

always

1. configure UDN for given namespace
2. create VM in that namespace - verify that primary network is UDN
3. go to "Add network interface" modal
4. in the dropdown UDN network is available and can be selected

Similar steps apply for CUDN.