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You can try, right click and select Copy Page then go to a new web document and Paste > Paste Page. Then see if you can add additional pages. If yes, then copy and paste your existing work into the...

No, the Supersite option is in the Web Properties > Web Export Options. But I just tried this out and I can still add new pages.

I wonder if your document has gotten corrupted?

If you create a...

Is your document a website? (Page Options > General > Document Editing...)

And if so, is your site a Supersite?