I have so much hair on my chest and belly, can I use it for a hair transplant?

Yes, it can be done but should you do it?  Body hair has a very short growth cycle, usually between six to eight months, which means that it not only will look like your body hair but it will be short (no more than one inch).  That does not give you either bulk or length which is a requirement for styling and value.

If you were very bald and have a poor donor density, then this hair could be placed behind the hairline or any on other part of the head except the first one inch of the hairline in order to add bulk to the hair transplants from the scalp. This is where the value is and if you happen to be very bald (a Class 7 balding pattern) then your chest hair might work.