You can back the up, but I don't think you can do exaxtly what you want to do.
First Quit Mail, then I'd backup these two Mail folders, by right clicking on them in the Finder, then choose Archive/Compress.
(Could be a different folder here if you chose such in Mail Prefs)
Right click on that Mail folder, choose archive, you'll get everything in the folder, and the folder itself in a file called Mail.zip, move it to a safe place, same for the Mail Downloads folder... only the plist is separate.
You can backup / archive the entire user/Library/Mail folder (as BD suggests) which will preserve your emails as you have them now, but in order to access them in the future you would have to copy your latest Mail folder to another place, then overwrite with your archived Mail folder, and view them as normal emails. You'd then need to swap the two folders over once again when you're done. Which is unwieldy.
The other way would be to set up another user account especially for archived email, copy your Mail folder into that account's Public/Drop Box, login to that user, move the Mail folder to that user's Library, and launch mail. You'd only need to do the Drop Box thing once if this is a one-off exercise, then you could see your archived emails as and when you want just by logging in to the other user and launching Mail.