LWes is right, its possible but its work if you don't have the right tools
Still, he looks prettty spiffy with what you've done. My only suggestion would be like Lwes said, try repainting so the whole body matches.
Nice job!

Saying there isn't room for another pair of arms is just an excuse, since I and several others have done it before. Search for "doppelganger" in the custom showcase. It's a bit of work but it's possible.
I also recommend repainting the figure, or at least the blue so the two pair of arms match in color, and they won't look so out of place.