The success of the head transplant surgery depends on the acceptance of the immune cells of the body on which the head is going to be inserted. Can stem cells from thymus gland and bone marrow of the head donor be transplanted into the body on which the head is going to joined, then those stem cells will produce lymphocytes which will accept the transplanted head. Can this be a way to increase the probability of success of the head transplant operation?
I think that immune rejection is not the only major problem in head transplantation.
We are quite skilled at suturing vasculature, but it's not fully solved yet how can you make new and proper connections in the spine?
Concerning your questions, if you just took these naive lymphocytes, they would probably get destroyed by the acceptor's immune system pretty soon. If this wasn't the case, the approach would already probably be in use for other transplantations too.
What you possibly could do is a bone marrow transplantation, where you would replace the bone marrow of the acceptor with that one of a donor. However this would cause the opposite problem, the donor cells attacking the acceptors body.
For now, lifetime immunosuppresion is the only working way to go.
To get a better idea about how exactly the mechanism of rejection works and the prospective future therapies, see for example: http://emedicine.medscape.com/article/432209-overview#a8