Put simply, you don't. Can't add menu commands to the default menu screen (though you CAN take away anything..)

I suggest a Skill called "Change Class", with its Type as a switch which activates a common event. Unless, of course, you want to build your own CMS...

Set that Skill as #0001 on the Skills screen, so it always appears first in every character's menu. Obviously, only allow it to activate in the Field.