Prop Ma'afu heads to the Welsh capital from Sydney's West Harbour Pirates, while Copeland switches from Championship side Rotherham.
The brother of Australia international Salesi Ma'afu, Campese is a six-cap Fiji international, while Copeland has impressed with an impressive 11 tries in 20 games this season.
Speaking about the signings, the club's forwards coach Justin Burnell said: "A big thing for us as a region and a team is we should be looking at a three-year cycle of establishing players that will be here to build for the future.
"Robin is a big, physical boy and Campese is an exciting prospect. I think they are more than capable of accounting well for themselves."