Affiliate software with external user db?
 

I have a somewhat technical question about affiliate software packages:

Are there any affiliate software packages that integrate with an external user database? That is, if I already have a user database (including usernames and passwords) maintained by a separate application, is there any affiliate software that will use this db instead of creating and managing it's own?

I have development resources aplenty, but not enough knowledge of the products in the marketplace. Is there anything out there that will allow this kind of integration?

Thanks in advance!