Can I get a transplant if I have a keloid scar on the back of my head?

In short. Yes you can have a hair transplant but your risk of getting keloid scarring is high. In the patients with history of keloid that I have performed hair transplants on, we did add steroids injections to the process, this lowers the risk but does not eliminate it. Please go to a qualified hair restoration surgeon who will examine you and advise accordingly. 

If you are prone to getting keloids you will have to understand any cut on your scalp may cause keloids.  You can still have a hair transplant (or any other surgery).  You just have to accept the risk of keloids.

If you have one Keloid scar then you are at a higher risk of developing other keloids from any wounds of the skin including the scalp. Hair restoration surgery requires many small wounds to be created and the possibility of keloid formation must be considered.  You may want to consider other options such as PRP, Laser Therapy, Medications, Supplements to support better hair growth.  Best regards, Dr. Weiss

Actually there will be almost no scar or obvious marks nor at the donor area neither at transplantation site with FUE  tech in normal person. But ıf you have a history of keloid scar , you should be carefull that when shave your hair with razor, you or others may only realize a tiny white dots on your skin when looked very closely..they are very tiny and invisible scars of punch insertions.Actually I have no complaint about those scars before but  if keloid regenerate and may turn  the white dots to bumping elevated scar dots, it may be a problem for you. You must not have a long, 2-4 mm widht pretty visible scar  FUT technique because of keloid formation risk .

I would have to see what you are talking about as keloids of the scalp are extremely rare. Most people have hypertrophic scars when they think that they have keloids

Keloids are a risk for any surgery in people who are prone to getting them.  In your case, you might not want to risk it, but you sould also get several opinions first, because a type of poor healing called "hypertrophic scarring" is often mistaken for "keloids."  Both can be managed with steroid injections and careful techniques, but if it is a true "keloid," any surgery will carry more risk.
FUE has also been known to create tiny keloids, but may be a more manageable option for you.  If you go to an expert hair doctor, they will know about this and how to manage it. 
Good luck!