Jennah
I'm not sure I fully understand your question. Are you looking to add signatures in an automated fasion in an ad hoc fasion, or did you want to design the forms with signature fields already on the PDF form?
Assuming you are using LiveCycle Designer to create the forms, you can ad signature field(s) to the form when you create the forms. When a user clicks on the signature field, they will be prompted to select a digital certificate to sign the form with. Note that if the end users only have Reader, the form will need to be Reader Extended (to enable the ability to sign a PDF with reader)
There are other third party methods to sign PDF documents such as CIC's SignIt.
here are some links that may be of use:
http://www.adobe.com/security/
http://partners.adobe.com/public/developer/security/index_security_partners.html
http://www.adobe.com/security/digsig.html
Regards
Steve