Customer Exit for service PO in ME21N

Dear All,

When I create service PO with the following step:

1. Create a purchase request (PR) with an item which item category ('D' - Service)

2. Input a Service number / External service no. in item's details

3. Create a purchase order (PO) with ref to that PR

4. Input an outline agreement number into the PO item

5. Copy service line item(s) to PO<b> only if </b>service number / External service no. are covered by the Outline agreement number.

I want know how to make a checking by customer exit in ME21N at step 5?

Thanks,

Well