As a circumsized guy, I will have to answer on general terms. You can wear an extender with the foreskin forward. The trick is to put together the extender in a length that - when compressed - the head of the penis is almost in front of the frontpiece. In that way, the noose or strap will not have to be so tight, allowing for some blood flow. That is important when you want to extend with the foreskin not retracted, as the tissue in the skin are in greater need of fresh blood, than the glans. When treating injured people, by using a tornique, it is important that the tornique are beeing released every 20 minutes maximum, to restore blood flow to muscles and skin for a few moments to prevent from further injuries, and then tighten again. When wearing the noose not so tigth, you are allowed to jelq some blood forward and restore some kind of blood flow to the foreskin and glans, and can wear the extender for longer sessions, but monitor your foreskin and glans from getting cold.
To make it more comfy, you may find some soft cloth, gauze, sports bandage to wrap around your penis before strapping in. Good luck.