The situation you have is a PDF that is being used for a project in knitCompanion. You have pieces setup against this PDF, and you want to keep those pieces. But, you also want to add pages to the PDF so you can create new pieces (say from a new clue in a Mystery KAL series) without settting up a new project.
This is very easy to do.
1. Open your project and go into Setup
2. Tap Append PDF
3. Select the PDF with the new pages you want to append to the existing project.
4. Tap Select (to add the entire PDF) or Partial (to pick just the pages you want to add)
The pages will be added at the end of the existing PDF for this project. None of your existing pieces will be affected and you can make new pieces directly off the new pages.