Modify>Convert>Poly-to-SubD

If it doesn't work try...

Polygon>Cleanup, in the options select clean non manifold geometry
Check for extra hidden faces, maybe duplicates, though clean up op usually picks up on these.
Delete all unused vertices.
Sometimes if the problem area persits, I find it easier to delete the surrounding faces, and recreate them using the append polygon tool, this will fix it.

Hope that helps