I have the exact same question but my dilemna is as follows: I have all my flights scheduled for the remainder of the year (LGA-YUL-LGA) every weekend until the end of the year, and will be missing only 1 segment for 50K, is it possible to change one of the LGA-YUL to a LGA-YYZ-YUL flight (as in adding a segment so I'm able to hit the 50k) ?
Could this be possible at the gate or counter at the airport? I am usually able to get on an earlier flight very easily when I present myself at LGA as the staff there is usually nice, but adding a segment is something I've never tried and could really help me.