If you've got the keyboard directly on the computer now, then no, plugging it into a self-powered hub won't change how much stuff you can plug into the keyboard. That's because the computer itself counts as a "powered" USB hub as well (quite obviously).
bruceb is technically correct ... however I'd plug the powered hub into the computer, not into the keyboard. That's because I like to move the keyboard around on my desk, and the more wires attach to it, the more immobile will it become.