Yeah - Merge is what you are after. You merge the loose disk into the bigger release.
Go to the Release Group page of the loose disk and tick it and press “Add selected releases for merging”.
Then navigate to the Release Group page of the main disk set and again tick and press the same button.
Now you want to change the selection to “Merge mediums and recordings”.
What I don’t know or remember is how to make sure it merges as disk 11. Pretty sure that is sorted out by track titles\lengths.
Best to just walk through the steps and see what happens. You can always cancel the edit if it is wrong. Or if someone else comes along with better advice
Thanks for the link to the doc. So the trick will be to first add enough CDs to the single CD release as to match the correct multidisc release, and only after that do a merge.
I just checked without merging with only one disc into multidisc and then you only get to see a notice/warning that you need similar number of discs, not showing a pre-result of the possible mess being created.
You need to select the Append mediums to target release merge strategy.
That’s only if you’re merging multiple individual discs together, isn’t it? It sounds like this case involves merging a stray single disc into an already complete multi-disc set (the “Kermit the Frog” example from the doc) which requires the “merge mediums and recordings” option.
You don’t need to actually add CDs but just turn the single medium into a standalone “medium 11”. You do that with the up/down arrows on the tracklist editor: