Existing user accounts.

I have a list of changing user accounts. I want to use this with PunBB.

The accounts consist of an e-mail and password. Additions and removals
to/from this list occur without PunBB notification. It makes sense to me
that allowing PunBB to authenticate these users would be best done by
allowing these users to login with their existing e-mail and password.

I would like them to be able to have a username, but not modify their
account settings e-mail and password. ie. Any modification would be
pointless, as I will not write code to update that information at source.

I assume this require PunBB modification, but I don't know how much.

I would appreciate thoughts on the idea, some startup information
and possible things to worry about with respect to PunBB internals.