ME21N user exit or BADI

Hi SAP Gurus,

I need help from you people, regarding ME21N.

In 4.7 Ver. after executing tcode ME21N, I had enter all required details for PO. for this we will give external order no. in Account assignment catageory tab.

but my client requirement is, while enterning the Order no. in Account assignment tab Warning message has to be display based on item no., Account assignment catageory combination.

For this I couldn't find any USER EXIST or BADI.

Kindly help me to resolve this issue.

Thanks and regards,

sreenivas