This is just too easy, you’ll be suprised, or maybe not. Whatever.
Let’s start by grabbing a widget. We’ll go here:
http://www.apple.com/downloads/dashboard/
And grab… iStat nano. Now don’t install it just yet. Click on the widget file and rename it to “iStat nano.a”. Finder will prompt you about the name change, just click “Use .a”. Now, once was a widget is now a folder. Inside the folder is all the files that make-up the widget, in fully-editable form!
After you’re done, to repackage the widget, just change the “.a” at the end of the folder name, to “.wdgt”. Finder will prompt you if you wish to change the extension, just click “Use .wdgt”. And you’re widget is now ready to be installed into dashboard. Just double click “iStat nano.wdgt” to install.