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