Go to Home-Setup->Modules/Applications->Sales Orders->Complementary attributes and add “New attribute” (Sales Orders can be called Customer Orders in your case)
Select type as a link to an object and fill out the rest of the fields like on the attached image
Then you will have a drop-down list of all Third Parties where you can select your supplier
The supplier will be displayed in the Customer Order
as well as on the Customer Orders list (as long as you select it)
