How to make 'Routing Attribute 2' field a drop down instead of a free text in the invoice header?

Summary: We want to make 'Routing Attribute 2' field a drop down instead of a free text in the invoice header since our invoice approvals are based on this field and most often users enter a wrong data which leads to lot of approval failures . Thus , making this field a drop down with the list of correct values would avoid this .

Content (required): We want to make 'Routing Attribute 2' field a drop down instead of a free text in the invoice header since our invoice approvals are based on this field and most often users enter a wrong data which leads to lot of approval failures . Thus , making this field a drop down with the list of correct values would avoid this .