You would need to have the capability to add SIP or H323 trunks to your Nortel. If you can set up a new IP trunk on the Nortel you should be able to route calls between the PBXs and use the outside lines on the Nortel from an Asterisk client phone.
Setting up a SIP trunk would be easiest.
On Asterisk you would need to setup something similar to this:
port=1720 (or whatever else you define on your other pbx)
Not sure what the configuration would look line on the Nortel. But you need a SIP trunk pointing to the ip address of your Asterisk server.
Note: this just establishes the connection you still need to define how calls are routed in your extensions.conf file.