I certainly read your PP discussion 3 times, but it is not really clear.
Let us go through:
APO-PPDS creates PREQ and its not converted into Purchase order.
This tells me that you have already a requisition and it is not converted into a PO.
What do you expect?
Do you think this will happen automatically? Have you setup auto PO? Or shall we call your buyers and ask them to covert the requisition to a PO?
Production happens in R3 in different plant and delivery plant is different in r3.
Are you really still stuck in a R3 system, the last R/3 Version was 4.7
Now what is trigger to create stock transport order from production plant to delivery plant.
Didn't you already have a purchase requisiton? what was the trigger for it? Or for what is the requisition which you wondered why it is not converted?
What are you actually doing in APO, don't you plan all this in APO? Isn't APO the trigger? Isn't this actually a question which should have been posted in the APO forum?