Yes they can :) there is a really cool standardbred called “bionic woman” who was a fantastic grand pre jumper. She just had a few quirks. I watched a Tennessee walker that was probably 15.2hh clean up a high jumpers class with style last year. There is generally just a thought that they would not be good at it. One of the old stigmas in the horse world that still hangs around.
They sure can! Same way you ask any other horse to canter. If they were never allowed to go above a gait in training then sometimes it takes a bit for them to pick it up. Try lunging him/her first and asking them to pick up a canter on the lunge line.