After a bit of experimentation, I managed to import a small DITA file into an unstructured FM file without ending up with a block of pure text. Here's how I managed it, though the results may not be what exactly you're hoping to do …
- open unstructured .fm in structured FM – let's call it unstructured.fm
- open DITA file in structured FM
- save DITA file as .fm – let's call it DITA.fm
- File > Import file to copy the DITA.fm content into unstructured.fm, keeping its original formatting
A visible mismatch, of course, but at least some clue as to the intended structure of the imported content. Step 2 could be the trickiest, especially if you're relying on someone else's source files.