Yeah,
Jürgen is right as always. I did have several clients running this process but not solely for having a separate PO for the freight. Mostly it was for the shipment planning.
In the case like yours my clients used the same PO with freight conditions assigned to the freight vendor (see Simar's reply above). Then they created a separate output type for the "freight PO". This program took the vendor no. from the PO partner functions (the client defined the special partner function for carrier in the vendor master data), the dispatch address from the main vendor master data and the rest as usual.
This client was SOX certified, so they did have very strict requirement of "three way match"; the auditors do accept such match against the material PO with freight reservation.
BR
Raf