UCD-SNMP-MIB support

Does SNMP4J support the net-snmp package of UCD-SNMP-MIB?
If not, what are the alternative ways to GET those/similar attribute values?

Sure, you can simply use the OIDs of the MIB module and build a ScopedPDU with type PDU.GET for them and the send that PDU with a Snmp instance to your Target SNMP entity.
See https://doc.snmp.app/pages/viewpage.action?pageId=42369028 for details.