People have trained their chickens to use pet doors, yes. How easy this will be depends probably a lot on your particular chickens. In some cases you may need to construct some training doors of different lengths so you can start with a short door (only bars the top part of the opening) and let them figure that out, then swap to a longer one that covers maybe the top half of the opening, and so forth. If you use a similar material (translucent if your petdoor is translucent, etc) this need not be terribly complicated, just make a flap of something suitable and flexible and temporarily screw or staplegun it up over the doorway. Once they are using a full-length 'mockup' flap happily, you can put in the real one. Again, with some flocks this won't be necessary, with some it will.
I use translucent shelf-protector material cut into strips to function like the strip curtains used for walk-in freezers and some animal housing. It was cheap. I trained the chickens by just putting up one strip at a time (not adjacent ones, either), I believe it has usually taken a pen maybe 3-5 days to cope sufficiently with that for me to put up the next strip. Works fine, but more wind does blow through than if it were a single piece of material. Other people use heavy canvas or burlap, or things like that.
Do remember this will not be even remotely predatorproof, so unless you choose to believe that you own one of the maybe three genuinely-predatorPROOF runs in all of North America (and I can pretty well guarantee you don't), if you rely on JUSt a pet door at night, it is definitely a greater risk for losing chickens someday. Of course there is no reason at all not to have a "real" popdoor that closes securely, at the same time as also having a pet flap in the opening.
Good luck, have fun,
Pat
