Can you use one's leg hair for a hair transplant, as a donor supply?

You can use your leg hair for donor hair supply theoretically.  The issue is the practicality of this method and how well it may grow.  You need to see a doctor if you are considering this type of surgery.  It is not a standard area of harvest.

Leg hair and other body hairs can be used to transplant to the scalp, however the best practice is to use scalp hair as the primary resource.  Remember that normally there is plenty of donor hair on the scalp for most people.  We only need to go to other sources in special cases.  It is very important for you to do everything possible to avoid accelerated hair loss.  Using your own body's growth factors with PRP (platelet rich plasma) and SVF (adipose derived vascular fraction) can make a huge difference in slowing the progression of hair loss and recruiting new follicular growth without surgery.  

Although technically you can transplant hair from one part of the body to anywhere else you have to remember that the hair in the area you're taking it from retains its original genetic predisposition. Because of this, hair from the leg would only grow as thick and as long as it does on your leg, it would not become head hair that grows thicker and longer than anywhere else on the body. Ocasionally we use hair from elsewhere as fill or to camiflouge a scar but that is usually all it is transplanted for.