I'm not familiar with a PS variant, but sssuming you have the various locations setup under one common supplier in P2P, the locations should be selectable by the end user in the detail of each line item on the requisition under "Contact". The same "Contact" field is available on the invoice for the invoice agents to select/change.
In all honesty, I believe the app selects the first location it sees on the list when defaulting the location for the line item. This would need to be verified by Suppor or your P2P Tech Lead. I'm also not sure if setting up Purchasing Units will help you filter locations by BU, but this can be explored with Support.