brick
As you have been told, SME does not support what you want (at present).
Some work towards that sort of feature has been done with samba & LDAP, but as far as I understand, they are both partial beta implementations and still need a lot of work to finish.
The feature ("pseudo" centralised login) has been requested many times before.
If you search (devinfo lists, bugzilla & forums) then somewhere in there was a post by Charlie Brady a few years back that explained there were many different files in SME that each had some pieces of information that would be needed to be transferred to other SME servers (for duplication etc).
It would require a significant amount of development work & skills from someone who has a very good working knowledge of SME server, plus testing the external interfaces etc.
It's Linux so theoretically anything can be done (coding wise), so such a feature in theory is possible, but considerable effort will be needed to implement it.
If you contacted Charlie directly and proposed to fund the development work required for such a feature, then the end result will or may (subject to do-ability) give you what you want, as well as being a great gift back to the community for all others to avail of.
Many good features in SME have come about because of direct corporate sponsorship for specific requirements.