Trying to encrypt external HD . GPT scheme required . Do not want to erase data
This might answer your question - though it scares me to death - https://superuser.com/questions/495672/how-can-i-convert-a-mbr-partition-to-gpt-without-loss-of-data

Alternatively, iPartition can do it - but can't handle Core Storage Volumes, so it's usefulness may be short-lived as more of these come into use. [Unless they release an update, of course]

Either way, I'd make sure to have a backup before starting.