If that's what you want to do, you can do it all with Finder.
Quit Mail, so that you have a stable environment while you're working.
In your library (example, disk/users/saxon48/library)
locate /Mail
then locate the folder for the account you no longer want. This will have Drafts, INBOX, Sent items, etc. in it.
Copy/paste that folder somewhere else (desktop, external drive, wherever). This step isn't really necessary, but I'd do it anyway -- it gives you a complete copy outside of your library, so if you screw up you can start over.
If you don't have a Time Machine backup, you might want to copy/paste your entire Mail folder somewhere else, just in case.
Now go to the INBOX of the account you're going to delete and then copy/paste (or drag...your choice) the contents of Messages (the .emlx files) over to the folder to which you were going to forward them -- presumably the INBOX for the account you want to keep.
Mail doesn't care how those .emlx files got there, so far as I've ever been able to tell. I've moved .emlx files between IMAP and POP account folders, and the reverse. It's all the same.
You can move over whatever else you want to move at the same time -- for example, pick up files from Attachments and copy/paste or drag them to the Attachments folder in the account you're keeping.
Delete the account using Mac Mail.
Keep the copy you made until you're certain that you've really done what you intended to do.
Unless I'm misunderstanding what you want to do, this would work. I've had to do it several times. There may well be a more sophisticated procedure, and someone else may post one. All this one has going for it is that it's simple and it works.
