This can be done by creating vouchers for the desired price plan.
Go into the price plan you wish to create a manual user for, allow it for both online purchase and vouchers. You may wish to do this for all your price plans, if you want to be able to create vouchers for all of them over time. This activation of price plans for vouchers only has to be done once.
When activated for voucher creation, go into Users->Order Vouchers. Order one or many vouchers for a particular price plan. you will receive an email with username and password, which you can forward if required. The system also generates physical vouchers as PDF files you can print out if desired, so they can be handed out to your customers, if you wish. There are also CSV files with the username and passwords created, in case you wish to feed a printing program of your own choice with the data to produce your own vouchers.