Good morning, gang.
Felicity: It may be helpful to know upfront that because you can't import these types of relationships, you also can't export them. This means that relationships you add manually will disappear if or when you batch export to do descriptive revisions or clean up.
We use two approaches for navigating this.
1. Use the general note field to share a 'See also..' type note; or
2. Use the Availability of other formats field to indicate duplicates or related materials elsewhere in our holdings.
Doing so means that if at any time the link breaks, the relationship(s) are still documented from a relationship standpoint.
For example, here's a record (
GA215-7-111) where the link related link isn't active but the note about the relationship between this photo and another keeps that information intact.