Diabetes will not prevent you from having a hair transplant.  Please talk with your doctor to make sure that your blood sugar is under control.  You must make sure that your blood sugar level is stable at least ten (10) days before the operation.  It is important to remember to take your medications on a regular basis and make sure you have a balanced diet.
Diabetes affects the healing process and because of this we want to make sure that you will not have any complications such as risk of infection.  One way to do this is to make sure that your blood sugar level is stable at least 10 days before the procedure.