How can i merge a copy of Exchange 2003 databases

Keeping a backup of your files, you might be able to just slot the files in, but you will need to have exactly the same names/SIDs, etc on your server.  

The way I'd normally accomplish this is to install the new exchange server on a different box and then move the mailboxes from 'server old'-->'server new' and then take 'server old'.  If server hardware is a limitation, I would temporarily use a PC as 'sever new' and then repeat the process once more recycling 'server old' back into the architecture after it's been reinstalled as 'server newest'

Finally, if you exported all users mailboxes to .pst files before doing the reinstall, you could then just have users keep them as archives and start over with empty boxes.  A little cleaning out of the box never hurt server performance.

If you just backed up the files and then wiped the whole machine, you might have some trouble to restore the DB...

Thanks. Unfortunately i slightly rushed into it. i exported 90% to pst files but the network contains macs using mac mail. I couldn't see an export facility. So i was hoping that somehow i could extract those mailboxes. What does exmrege do? (or something like that).