I've been investigating doing this for my current employer. We currently capture customer signatures on NCR forms and I'd like to eliminate the rigid layout, dot matrix printer and expense involved.
From what my research shows, there are companies that sell gadgets for electronically capturing a signature (like the package delivery guys carry around with them). It should be possible to capture a signature and store it in a container field in Filemaker. Note that I said "Should". I haven't actually done this yet.
Do a web search under "signature" or "signature capture" and see what pops up.