The FXO port would connect to the ATA from the Norstar, acting like a CO coming into the Asterisk box. I haven't worked in Asterisk, but I don't see why you couldn't use all of the features you want through an FXO port. How else would Asterisk provide integration with message waiting lights etc.

If you do need to use a feature that can only be used through an FXS port, perhaps you could connect an unused CO port on the Nortel to an FXS port and have the Asterisk dial in that way.

I would get a card with at least one FXS port so you have some options to play with.