My donor area was over harvested. Can the donor area be filled in with beard hair?

 Yes, beard hair is a great option to restore
any scar or defects on the donor area of scalp.

One of the reasons that beard and mustache transplants are possible is because facial hair, particularly hair that grows around the chin, is actually quite similar to the hair that grows on the back and sides of the scalp. Body hair, however, is distinctly different. All hair follicles follow a predictable growth pattern that includes an anagen phase, where hair grows normally, and a telogen phase, where hair that has stopped growing is shed to make room for the next cycle. In the follicles on the scalp the anagen phase can typically last for years, allowing the hair on the head to become quite long and full.  However, in the follicles that occur elsewhere on the body, the anagen phase generally lasts only a few months, while the telogen phase may last up to a year. This shortened growing period and extended dormant period explains why the hair on the head tends to be much longer than other hair found on the body. Further, the hairs on the body tend to be either very course or very fine and non-pigmented. Neither is a good substitute for the hair on the scalp.

Moreover, any time hair follicles are removed for transplantation some form of scar will be left behind, whether it is the thin linear scar of an FUG procedure or the hundreds of tiny circular scars left behind by follicular unit extraction (FUE). While steps can be taken to minimize this scarring, the removal is still best done in an area where surrounding hair growth can fully conceal those scars. Since hair coverage is far less dense on the body than it is on the scalp, and because the areas that would be affected on the body are generally more visible than the back of the head, the final result is not likely to be at all satisfactory.