Bank Document type in Purchase order

Hi ,
User need to attach Letter or credit to purchase order . Generally when user creating the PO from the Purchase order menu - When user click the New , there is an option to select the bank document type – LC in the general tab .
But when user created the PO directly from the Vendor master ( all the vendor information filled ) , there is no option to select bank document type ?
Can you tell me how to over come the issue ?