Yes you can do that. Once you have all your data migrated to the first GPT you can prepare the other drive and create the raid 0 array in irst without loosing any data. You can also do other things as it creates the array in the background but I'd let go overnight to be safe.
As with any of these things. I'd backup to an external drive before making the raid 0 array as an extra precaution. Murphy's law and all..