You will need to have a case logged and escalated with McAfee Technical Support to get this done.
For contact info in your area, see: http://www.mcafee.com/us/about/contact/index.html
I request you to please contact the Customer Service at 1 866 622 3338. Give them yoru Grant number and say that you want to MERGE your accounts.
You could also do this yourself using the Security Centre, However, For the first time I suggets you to call up the customer service.