is it possible to sign a pdf using nothing but xojo?
That one is in “the list” … but not in the current release. The steps involved (AKA code) in the PDF creation are not the same.
It is not built-in into Xojo.
You can of course relay on a command line tool to sign your PDF.
Or check out our MBS Xojo DynaPDF Plugin in the Lite or Pro version to sign the PDF.