How do you insert an electronically signed report as an appendix into a main report which will be signed electronically at a later date?

You can add it as a file attachment. Exactly how you do this depends on the version of Acrobat you're using, but if you open the Attachments panel, you should see where you can add a file.

You can also use Portfolio feature in Acrobat which manages attachments for you in an intelligent way.