Can surgeons create an underboob if one doesn't exist?

With proper placement of the implant, the skin will stretch over time giving nice fullness beneath the nipple.  It is a standard part of the breast augmentation. 

Its difficult to say without seeing your before pictures. Please consider reposting your question with photos.

Yes it probably can be done with implants but seeing a picture of your breasts would be helpful. Best to visit with a board certified plastic surgeon.

Assuming I understand your question accurately; yes.  Patients who have a short distance from the areola to their inframammary folds can achieve a greater distance/roundness in this area with breast augmentation surgery. As you state, the lower breast pole skin can be “stretched” and/or the upper abdominal wall skin can be recruited to increase the distance from the areola to the inframammary folds.  These maneuvers are routinely performed for patients who start out with constricted/tuberous breasts. Doing so, serves to round out the lower poles of the breasts  and better position the nipple/areola complexes on the breast mounds. When the time is right, you may benefit from in-person consultation with well experienced board-certified plastic surgeons  for  a more individualized/precise response. Best wishes.

I am assuming by under improve humane the lower pole of your breast that is the breast tissue that runs from the bottom of your areola to the fold under your breast.  If this is the area of concern to you then yes breast augmentation can fill the lower pole of the breast and create a nice curve below the areola and centralize the nipple areola on your chest.  Please see a board certified plastic surgeon.