Personally I don't have any experience with nortel, but there are several ways you can connect with external PBX.
One could be using fxs extensions from the nortel PBX and using them as trunks from Elastix. For example you can have 10 fxs extensions, that would became 10 fxo trunks for Elastix. By interconnecting the PBX in this way you would have the disavantage of only having 10 concurrent calls between the 2 PBXs
Other option could be to use E1/T1 channels to interconnect the two PBX. Similar solution than the one above but you would have more chanels for the interconection.
The last and the best option would be to interconect the two PBX with VoIP. The best solution would be SIP, but it can be done with h323 too. If nortel support IAX2 (witch I don't think it does), it could be even better.