To use existing PDF as a background for your imposition, click background.
When a background PDF is used, the sheet size cannot be changed - it is taken from the background PDF (its Crop size).
Name and number of pages is shown in the main dialog. When the name is ALT - clicked, the complete path to the background PDF is shown.
If the PDF has more then one page, these are repeated as necessary (1-2-3-1-2-3). You can use the background option for Variable data printing. Put the variable data into one PDF, the rest into another and then use it as a background for the first one. Then make sure that "PDF optimization" is ON at your RIP (e.g. CREO RIPs) and it will RIP the pages very fast because it will recognise that the background repeats.