I believe this would go against some of the intrinsic capabilities of synchronization.
For example, you can't perform an update/insert sync (upsert) based on a seeded SQL field. The reason being is because you can't insert the value; it has to be auto-generated on the db side.
While it makes sense to use it for looking up if a record exists, it can't be used for inserting records. Ultimately, the Excel Importer needs both to function in its consistent manner.
Solution: As you probably have guessed, you'll more than likely have to mimic that column/attribute, and use that for synchronization. E.g. use an on-create event to copy the autonumber into an int attribute.
Side note: Even in MS SQL (and I assume other platforms), you can't Synchronize unless you explicitly set some sort of "Identity Insert Property" on for the sync.