You can add relationships to other recordings (not from the release) from the relationship editor by clicking Add relationship to work (which mirrors what happens from work/edit). In work/edit, those show in the list of relationships being added, but do not show later if you open the edit page again (because of cardinality). In the release editor though, they just don't show at all - but the relationships are submitted.

This should probably just mirror what happens in work/edit.