MBR and GPT are two different partition table types, also different boot environments - I dont beleive it is possible. You may be able to take the main OS volume and cloning that to a GPT partition, booting from the windows DVD in UEFI mode and running a boot repair.. it MAY find the OS and setup the boot portion of the disk for you?
I have tried this previously. What I did was start a enw UEFI install (note the board had to be in UEFI only mode) which created the partitions for GPT. Then close the backup to the main system partition and run a boot repair?
It would be good if there was a utility for like GPT conversion.