I concur with Richard. Unfortunately this happens quite routinely at the Universities and federal institutes (it appears that they do not want eternal post-docs) and after two stints you begin to have problems with your permit.

I don't think that your University/institution would be willing to endorse the application for a 5 year B permit because of the nature of the post-doc contracts here (short term renewable).

After 5 years, why not apply for a partner dependent permit ?..... and forgo the hassles of dealing with all the crazy admin. involved