As you know, in the importing progress, you can specify single page, range of pages or all pages to be imported to the PDFPO. In this way, you can choose to split or merge PDF documents. If to split, you can import (split) a PDF document to multiple page files, if to merge, you can import (merge) different PDF documents to a file.
If you are importing pages for merging PDF files, you can repeat the above 2-4 steps to merge the page(s) into a same PDFPO file.
If you are importing pages for splitting a PDF file, you can open another PDFPO file, and then repeat the above 1-4 steps to split the other part of a PDF file into it.
For details, see Merge Example and Split Example.
Yes, you can use Foxit PDF Page Organizer or PDF Viewer without the support of Acrobat Reader.