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
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.