Please see if the following articles are helpful:
Oracle provide package procedure PO_CUSTOM_XMLGEN_PKG.GENERATE_XML_FRAGMENT to generate the custom tag for PO Output for Communication report. This tag can be used in XSL Layout to display required details.
For all your above requirement you can create the custom tag using above user hook those new tags will be available in view PO_HEADERS_XML , PO_LINES_XML etc. based on your customization.
For the signature, I am not sure its digital signature or manual. For digital signature we have added the signature as image and based on Employee name we are generating image name dynamically and calling is from XSL.
As Brajesh pointed out there is a custom hook.
See this note on My Oracle Support: Custom Hook For Enhancing Purchase Order PDF XML Generation Logic (Doc ID 1505737.1)
Make sure you put FND logging writes in your code, this will help you debug as you go along.
If you are 12.1 make sure you have the patch that's in the note. For 12.2 you should already have the package ready to use.