I would use the Nokia desktop software to transfer everything from the N91 to Outlook, and then use Desktop Manager, included with the 8700g to continue the transfer from Outlook to the BB.
We use nokia pc suite to transfer contacts from the phone to lotus notes address book and then do a sync and the contacts wireless transfer over the BB.
If the contacts can be copied onto the simcard then all you need to do is insert the simcard into the BB and copy the contacts from simcard to BB.