You did not overlook anything. We do not have this option yet.
There is not a single 'XML format'. XML is a formal standard on how to use tags only – there are several ways to store data in XML. Anyway I agree that at least we should be able to import the .xml format that we export ourselves. I think this was a repetition of a previous reply …
Are you aware of any approved or 'de facto' standard of XML data storage that we should support?
I do not know 'Basecamp data' .. details .. link?