Hi Niten,
The Lithium Platform supports the concept of what's known as Message Metadata, which allows you to associate custom key/value pairs with a message. You may have even seen some of these REST API calls while browsing through the Javadocs. Unfortunately, the ability to add new Message Metadata is not currently available through Studio, so you'll need to work with our Professional Services team in order to get the initial setup completed and make the form changes to add the field to the TKB message editor.
If this is something you're interested in pursuing further, I'd suggest filing a case so that our Support team can put you in touch with a member of our Professional Services team.