Is it possible to reverse orthodontic premolar extraction?

No!: Once the teeth are gone , they're gone!

Huh? and no: Orthodontists typically do not extract teeth-- they have dentists or oral surgeons do this. If, you are asking, can you put a tooth back in that has been taken out of your mouth-- no. You can have implants to replace lost teeth, but you can't 'reverse' an extraction. My guess is, if the orthodontist had the tooth removed, it was for good reason. I would discuss your concerns with your orthodont.

Why do you ask: Assuming you're under the care of a qualified Orthodontic Specialist, discuss this with the practitioner. If you're seeing a Generalist, ask for referral to an Orthodontist. Are you asking because you're concerned about changes now that you are in the midst of treatment, if so discuss treatment goals with Orthodontist and also possible solutions.