After some research without any answer to my question, if it's possible and does it make sense to sign an E-mail with PGP and s/mime (?), I'm hoping to get an answer here.
I know that, PGP and S/MIME fulfil the same purpose. They create the signature with the same cryptographic utilities, but manifest a different format. The question about the differences of this two tools were already asked here: How does PGP differ from S/MIME?.
Far more clients are compatible with S/MIME out of the box. Is it possible to provide a PGP and a S/MIME signature per E-Mail, so I can satisfy PGP and S/MIME users?