There have been cases where I have a box set that is not found, and I have to manually enter it. But the box set is a collection of several other single albums. It would have been nice to be able to select the single disc and copy it to the new box set. Alternatively, if I could select the tracks with the checkboxes, then do copy/paste to another album.
What you could try is making a duplicate of the single disc and then move the duplicate into the box set.
If you have the loose albums as loose albums in your collection, you can click an album, edit it, and just modify the box set field and enter the box set title.
Each album with the same box set filled into the box set field will be part of that box set.
Try it with one, to see if you like the result.