The best way to transform actual human handwriting into something that the AxiDraw can plot is to capture that handwriting in real time. In real-time capture, you use your computer to record each stroke as it is written, and then save the resulting set of paths as a vector artwork file.
This page discusses some approaches to real-time capture, as well as some alternatives. Naturally, these same approaches can be used for capturing sketching and other non-handwriting types of artwork as well.
Capturing a signature with jSignature
For very small amounts of text — for example a single signature — a convenient software tool is jSignature. jSignature can create and save an SVG file which can be opened in Inkscape. jSignature can be used from your computer, smartphone, or tablet.
To use it from your computer, sign your signature, and then select "SVG" from the "Extract signature data as" pop-up menu. It should then display your signature as an image below that pop-up menu. Right-click on the image, and select from the menu there "Save image as..." (the exact wording of this option will vary depending on your browser), and save the result with a name like "signature.svg". You can then open up this SVG file from within Inkscape and print to the AxiDraw. For other platforms (smartphone and tablet), you'll need to send the signature to your desktop computer, by a means such as e-mail, before proceeding.
Capturing with a graphics tablet or tablet computer
Using a simple graphics tablet
One solution for capturing writing is to use a dedicated graphics tablet such as a Wacom Bamboo or Wacom Intuos Draw. These are relatively inexpensive (< $100) input devices with a stylus. You can use the stylus just like a pen, within almost any application that supports drawing and creating vector graphics. (Be sure to use a vector graphics application such as Inkscape or Adobe Illustrator, and not a bitmap graphics application such as Photoshop.)
While you can certainly write freehand on a tablet like this while watching your screen alone, another approach is to trace over a piece of paper. You can attach a piece of paper to the top of your graphics tablet and use the stylus on top of that, to trace handwriting or other artwork.
Using a high-end graphics tablet
There are also higher end graphics tablets such as the Wacom Cintiq that have a full touchscreen display, in addition to the stylus input capability. These tend to be considerably more expensive, and you may wish to consider a general-purpose tablet computer (e.g., iPad) instead.