Your best bet is just to make a second reservation for the BKK-DOH. It will be a little more expensive in terms of avios and taxes than a single through ticket, but single through tickets are not available. An alternative might also be to look for other dates when through tickets are available (if any) and change to one of those.

As an aside, sometimes even if BKK-DOH and DOH-YUL were available as single sectors, you might still not be able to buy one ticket or redemption BKK-YUL. This may be because the airline wants to save availability for DOH traffic rather than connections.

QR should still check you through on 2 tickets though, provided the gap between flights is suitable. Or you could add your own stopover.