Can body hair be used for a hair transplant to fill my beard?

Body hairs can be used as donor hair for facial hair restoration.  However scalp hair is usually the best source of donor hair.

BHT, or body hair transplantation, is a technique that has become a more popular procedure, due to the large potential number of hairs that can be obtained.  My preference is the use of beard, followed by chest and or back hairs, where I have harvested as many as 3500 grafts over two days.  These are definitely sophisticated challenging cases, but these hairs can be used not just for the scalp but also for the beard.

I use only head and chest as BHT donor sites for beard restoration . Armpit is not suitable area for donation both for sweat gland structures and donor area hygiene problems 

Yes, body hair performed by a skilled hair transplant surgeon can look natural. A great first step would be an online consultation. Often you can submit photos to top hair restoration surgeons for a complimentary evaluation. This should help lead you in the right direction. Having multiple consultations with different doctors cannot hurt. 

The best site for beard hair transplants is from the beard under your chin rather than use ofher body sites