I booked:

LAX-YVR-YYJ-YVR-YYZ-EWR//LGA-YOW-YHZ-YYT-YUL-YTZ-YUL-YYC-YVR-SFO by calling in. They had to send it to ticketing because there were too many flights

Ticketing got me exactly what I wanted, but I've found for anything even remotely "complex", the front-line agents aren't always capable.

But all in all, if you just verify fare classes and total cost, you should be able to do it. If they can't, ask them to send it to ticketing, but only if you're sure you're right. For "last minute" trips, ITA may show fares you can't actually book at that time.