How to transform messages using BRF+ ?

It has been some years since I used BRF+ last time. Maybe you can give a quick advice on how to build a simple BFR+ function that should follow this logic:

• get a message with structure BAPIRET2 as an input
• if the message is found in a decision table its type should be changed to 'E', remaining message is returned without a change
• if the message is not found it should be returned without a change

Using only a decision table I cannot alter the message. Should my function use a rule that somehow uses the decision table? Or which artefacts do I need to get the desired result?