You may use, modify, and redistribute this applet under the terms of the Gnu Public License. That is, it's open-source. Therefore, if there's a change you'd like to see, your best bet is to learn java, or find a java programmer, and download the package. You may also download the package if you'd like to have this applet on your own machine instead of needing to visit the web site.
For more information on metric conversions, go here... No, just kidding. Try the National Institute of Standards & Technology, hopefully a good starting point:
They provide an exhaustive(ing?) list of conversions in a .pdf file, "special publication 811". (if this link yields a bunch of garbage text, try holding the shift key down while clicking on it)