Page Numbering; Two numbering sequences

I'm not sure what to call this issue. I have 200+ page document with the page number at the bottom of each page. When I get to the last 20 or so pages of the document, I want to add a header that will have a different page number, while preserving the main page numbers at the bottom of the page. It is kind of a document within a document. The last 20 pages should have the page number at the bottom which continues from the main document. But I want to put a second page number within a header that designates page numbers just for this secondary document (starting with page one). I haven't been able to figure out a way to do this. Everything I've tried hasn't worked. I thought maybe I could do it with chapter numbers in the header, but I can't find the way to reset the value to start at page 1 (chapter 1) without changing the main page number.

Is there any way to do this? The only alternative I can see would be to discontinue and restart the header on every page and insert the correct number. I would rather not do that because it messes up the justification.

I appreciate any help anyone can give me.