Modify LOV for "Supplier Name" & "Site" fields on Non-Catalog Requests

Can the Supplier Name and Site LOVs on a “Non-Catalog Request” iProcurement Requisition be customized?

When a Non-Catalog Requisition is being created, we would like to prevent the selection of a handful of Supplier Sites linked to automatic generation of Purchase Orders.

I'm considering a NOT EXISTS clause

NOT EXISTS

(select      'X'

from        po_ga_org_assignments_v       org

inner join  po_headers_all                poh

on          (org.po_header_id = poh.po_header_id

and not      (nvl(poh.closed_date, sysdate) < sysdate  -- Outside Effective Date range of Contract

or            nvl(poh.closed_code, 'OPEN') = 'CLOSED'  -- Contract is Closed