In theory you can, and occassionally we do. But it's usually a last resort.The best hair for scalp hair transplants is the scalp, followed by the beard hair, chest hair.The chance of survival with leg hair is very low compared with scalp hair.
If your request is to have thicker hair, and you care to send photographs, we would be happy to analyze them and give you an opinion. Generally leg hair is not dense enough to be used for a hair transplant.best of luck,