If you would like to use your own Purchase Order numbers in Syrinx you can enable the "User Entered Order Numbers" option on the "Orders" tab of Syrinx SP Configuration\System Options\Purchasing. If you would still like the option to use the Syrinx auto generated purchase order numbers with the option to use user entered order numbers as/when required, you simply need to enable the "System Supplied Default?" option when enabling the "User Entered Order Numbers?" option.