The crown isn’t a styling area, mostly just need the look of fullness over there. It’s common just to get that area micropigmentation. So why not just use some body hair to fill it in? I know there’s a reason why, otherwise it would be done more. But what is it?
Body hair donor sites have problems with them as follows: (1) they grow to a limited length, (2) they only grow about 6-8 months and then fall out, (3) their texture (bulk) is less than most scalp hair so that it takes many more to be equal to a scalp donor hair and (4) half of all of the transplanted hairs are in telogen (sleeping) half of the time which means that for every 10 hairs that are transplanted, only 5 are growing at any one time. In conclusion, it is not practical and very expensive. As for hair elsewhere in the body, beard as a donor source is good but not for the frontal area and only when you run out of good scalp donor hair. Neck hair is not permanent hair so it should never be used as a donor source.