Jens,
To synchronize data with the external system you could modify the default MembershipProvider as well as modifications to the ProfileProvider.
I don't currently have a working example of building your own ProfileProvder but it is implemented in the same way using the Provider model. You can either modify the out of the box providers to sync with the other system or make your own. I was suggest however to make your own (at some point) so that you can avoid upgrade issues.
Hope this helps.