Workers who hold H-1B visas can only work in the activities described in their petition for the H-1B. If the contractor you're planning on hiring is going to do the same work for which he or she petitioned, all you have to do is fill out a Form I-129. However, if the H-1B visa has been "sponsored" by a particular company, this may not be transferable. Speak to an immigration attorney to get the guidance you need.