[fetchmail]changing local domains for multi-drop mailboxes?

I fetch my mail from an ISP (foo.de), to a local network
(foo-intern.de) with fetchmail (currently for each user separately). 

It would be easier not to manage any box at the ISP and fetch the mail 
using a multi-drop mailbox. The MTA at foo-intern.de now needs to know 
foo.de as a local domain. This setup works, but hten I am unable to
send mail from foo-intern.de to foo.de (it gets delivered locally).

For user names it's possible to differentiate the here and there
user. I would like to do the same with domains as well:

poll pop.provider.net no envelope localdomains foo.de=foo-intern.de:
               user maildrop with pass secret1 to esr * here

Then its not necessary that foo.de is known as local domain (to the
MTA) and it's possible to send mail from  foo-intern.de to foo.de.

Is there any way to accomplish this with the current fetchmail? Or are 
there other/better ways to do this?

Thanks