Adding attachments to a document that had been signed

Hi,

is there a way to attach an additional document to the agreement after it had already been signed? I need an invoice signed immediately, and attachments to be added later.

Or to attach a document at the point of signing? I read elsewhere that there is a field that I can add to the document, but I can't see it. If I need an upgrade to do it, what do I need to upgrade to?

Thanks,

Epi