Place the PDF into a new document the size you need for 9-up and make 8 copies and place them.
I'm sorry, I don't actually know what you mean. I tried just copying the design to fit nine on a page in Indesign, but that didn't work out right.
If you make a new document and go to File>Place, you can navigate to a saved InDesign file and place it as a single image the same way you would place a tiff, jpeg, pdf, psd, etc. If you edit and save the original InDesign document that you had placed in the second document, the links manager of the second document will show the link as modified, which you can update the way you would update a modified graphic of any other sort.
If you duplicate the placed image nine times and space them correctly, you get the nine-up document you can print. Just take into consideration whether you need marks of any sort, which can be made manually, or you can select the image or images and go to Window>Utilities>Scripts and run the CropMarks.jsx. If you mess with it a little, it will make sense.