You don't have to go to Canada to interview for NEXUS, there are a number of enrollment centers in the United States, though all are relatively close to the US/Canadian border. I interviewed at Boeing Field in Seattle, registered my IRIS scan with CBSA in case I fly to Canada some time soon when I had other business at YVR, and registered my fingerprints with Global Entry on a connection in MIA.

Global Entry members who are not members of NEXUS can use NEXUS lanes when entering the United States (all passengers in the vehicle must also be Trusted Travelers and present their GE/NEXUS/SENTRI/FAST cards). Only NEXUS members can use the NEXUS lanes when entering Canada.