Does hair from beard as donor area grow back in the donor area ever again?

Thanks for your questions Izik.  Not all beards are created equally so of course a proper evaluation of the quality and density of a patient's beard as a resource is important.  If it is determined that enough beard hair can be harvested to make a meaningful change while being able to still maintain an acceptable not overly depleted appearance in the beard region then proceeding in this way is feasible. 
It may be prudent to take a conservative approach with an initial beard harvest to see how your skin reacts and heals and ultimately gauge the viability of a beard transplant growth as it pertains to your scalp.

Hello, and thank you for your question.
While the beard is a viable area to extract from for a hair transplant, the entire follicle is being removed from the area, and therefore will not grow back. If you feel this may be an issue, extracting from that area may not be the best approach to take. 
Gary Linkov, MD
Hair Specialist
Manhattan

Beard hair comes from hair follicles. It’s best to think of these structures as tiny organs. Once they are harvested through donor extractions, they are gone. And a newly generated follicle will not take its place. 
What you are noticing with the beard area looking “pretty bad and thin” has to do with the approach used to collect the follicles. At my practice, the Dr.U Hair and Skin Clinics in Los Angeles, we harvest grafts diffusely, instead of concentrating on extracting from one area before moving on to the next region. A diffuse approach helps to leave the appearance of the beard region looking virtually untouched. Below is an example of a patient from my clinic who was able to sport a short, stubble beard following his body hair to head transplant. You cannot detect any unusual regions of emptiness or sparseness, even though he had a large number of beard hair follicles extracted.