Until recently, partners that had been created in the master record of a one-time customer could not be copied to the sales document. This has now been made possible
You want to deliver to all one-time customers with the same forwarding agent. You maintain this forwarding agent in the one-time customer master record of the partner. When you create an order with this one-time customer, the forwarding agent is automatically proposed as the partner.
In the standard version, the maintenance screen for partners is greyed out in the one-time customer master record. You must change this in Customizing for the customer master record in order to be able to maintain partners
IMG: Control -> Define account groups and field selection -> One time customers -> Sales data -> Partner usage group.