Option #1: You can manually add a paperback using the "Add new book" button on the Books page. Be sure to click on the Details tab and add all of the data you have about the book including the ISBN numbers and the ASIN.
Option #2: If you have a KDP revenue report with the paperback included, then upload the file manually using the "Upload income report" on the homepage. This will add the books to AHS but you still need to assign the paperback to its respective eBook. Go to the Books page and look for a message near the top of the page that tells you if you have unassigned paperbacks. Go ahead and assign them and save the changes. The sales will now show. NOTE: AHS currently requires uploading any non-KDP file (i.e. Shopify, Nook) twice whenever a new book is launched. The first time lets AHS know the book exists and the second upload pulls in the paperback's sales/reads data. This only needs to be done when you're a new customer and/or when you launch a new book with a paperback version.