How to search a control ID in a resource dialog (without redefining its controls):
DEFINE DIALOG oDlg RESOURCE cResDlgName
ACTIVATE DIALOG oDlg ;
 ON INIT ( lFound := LookForControl( oDlg:hWnd, nCtrlId ), oDlg:End() )
...
function LookForControl( hDlg, nCtrlId )
 local hCtrl
 while ( hCtrl := GetWindow( hDlg, GW_CHILD ) ) != 0 .and. GetParent( hCtrl ) == hDlg
   if GetWindowLong( hCtrl, GWL_ID ) == nCtrlId
    return .T.
   endif
   hCtrl = GetWindow( hCtrl, GW_HWNDNEXT )
 end
return .F.