Ha! My co-worker made one of these for the same reason. Some places still require "wet" signatures, and some people (us) like to stick it to bureaucracy and prove that they can't tell the difference.

For a one-liner, if you use any tool to generate a realistic version of your signature, or do what I do and actually scan a version of your signature once, and then have that as a PNG that you can drop on any PDF of your liking, here's a line to make it look nice and "scanned":

  convert "$1" -alpha Off -density 150 -colorspace gray -blur 0.5x0.5 -rotate 0.4 -level 40%,60% "scanned-$1"

Or for even more noise and poor quality:

  convert "$1" -colorspace gray \( +clone -blur 0x1 \) +swap -compose divide -composite -linear-stretch 5%x0% -rotate 1.5 scanned-$1.pdf