Adding pages to existing pdf file
You could use pdftk from the commandline:
pdftk A=bigpdf.pdf B=insert.pdf cat A1-180 B A181-end output output.pdf
Or if you want a GUI you could use PDFsam (PDF split and merge), PDFjam or jPDFTweak. (PDF Shuffler has already been mentioned.)
PDF-Shuffler 
PDF Shuffler is an ideal application for this. It allows you to import several PDF files and rearrange the pages. You can also delete the pages you don't wish to keep in the output PDF.
