How to create new extension field in message class (ID 382)

Hi,

We have a requirement to increase the length of OOTB field named "Content" in the RFx --> Discussions -->New Message from 2000 to 4000. Here I am attaching the screen shots. Since we can't increase the length of OOTB field, I thought of hiding this field and create new extension field.

But while creating new extension in the Setup --> Extension Definition, I couldn't find class Message (ID 382). Please let me know how to do this.

Thanks,

Srini