I will probably create a new class of ships in
names.cfg for this unusual ship. Something like
Type22=Seaplane Tender
Unlike Nisshin, which in the game acts as an Escort carrier, this seaplane tender doesn't spawn planes as it was more of a servicing and repairing asset, so I think a different class makes sense.
If it is a new class, it won't appear in any campaign without editing a few files, so I thought about doing a set of missions that only needed to be added to campaign.cfg. I think it's the easiest way. I spent today doing a set of missions in ME to include the ship in a campaign. They're based on the Akitsushima TROM in combinedfleet.com:
I've never done anything like that, so I still have to test it and be sure that it works.