How to create custom tye_lookup_code in po_headers_all table in Oracle Purchasing

I want to know how to create a custom document type/po type like STANDARD, BLANKET, etc in Oracle purchasing.  I went and edit the look up codes for look up type 'PO TYPE' by navigating to set up --> purchasing --> Lookup Codes.  But from UI, I'm not able to edit the page. So I wanted to know, does Oracle allows the user to create a custom PO type? If so, how do i create such one? 

thanks

sen