Exit for Document Type(RPCAP-DOCTY) field validation in 9KE0

Hi,

I would like to validate screen field Document Type(RPCAP-DOCTY) (after entering value in screen)  in 9KE0 transaction. Please suggest if there any exits or any other way to do.I searched exisitng custome exits but nothing is suitable. Please suggest.

Thanks in advance.

Sunny