You can use Mapsofts DocuMerger tool to merge material from an existing PDF file into another PDF file. It is different from the Mapsofts PDFBuilder tool as PDFBuilder joins individual files together, one after the other to form a new composite document, while the DocuMerger merges a selected page from one document into another document.
The document from which material is to be extracted (the source document) can be any length: but only one page can be extracted from it at any one time for merging into the target document.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.