I added these three lines at the end to PDFCREAT.PPD, just before the comment '*%end of PDFCreator PPD File', to obtain two trays:
*% == Paper Trays =============================================================
*InputSlot Tray1/Tray 1: "<</MediaPosition 0 /TraySwitch false>> setpagedevice"
*InputSlot Tray2/Tray 2: "<</MediaPosition 1 /TraySwitch false>> setpagedevice"
Tray numbers will then be 257 respectively 258. /Mediaposition tells which slot in the 'Printer' it is, /TraySwitch false tells not to switch trays when a tray runs out of paper. 'Tray 1' and 'Tray 2' are the visible Tray names in the bin selection popup menu. The meaning of 'Tray1' and 'Tray2' could not be found by me (perhaps it should be just '1' and '2')