Anybody know how to do it?

You just add the line under the mail section.

[Mai] Mail Root Directory=path

Just make sure to do it while Opera Mail is closed and move your mail folder to that spot before you start Opera Mail.

In other words, opera:config is an interface to the settings in operaprefs.ini. So, you can just use opera:config in Opera 12 to see what you have to add/set in Opera Mail's operaprefs.ini. Or, you can just add opera:config back to Opera Mail.