Is it permissible to wear hair extensions for the purpose of treatment, improving psychological well-being, and stopping hair pulling, knowing that hair is still growing?

Joining (hair) which is forbidden—even if for an excuse—is what is done with other hair. As for joining with something other than hair, it has a different ruling. Seeking healing is not done through something forbidden, because Allah has not placed the healing of the Ummah of Muhammad (peace be upon him) in that which He has forbidden to them.