In the early days Nortel did dabble a bit with third party SIP licensing but it didn't get much traction. If your system has those kind of licenses installed already, you might be able to get them working once you put SIP firmware into them, but if you don't, your out of luck with a system that old.
You could look into replacing the 400 with an emetrotel UCx. They'll work on that ok as will all your existing Nortel phones.