How do I transition from a MobileMe account to an existing iCloud account?
Here is a fantastic article on moving to iCloud. http://support.apple.com/kb/HT4918

And, yes, you can keep your current @me.com address. Instead of being associated with MobileMe, @me.com email addresses are now iCloud (and MobileMe user will be migrated over), so the email address that comes with iCloud is @me.com. Again, I believe you are able to keep your current @me.com address.

Unfortunately, merging wouldn't be possible.