Messaging with Rails and Mailboxer

The tutorial goes along way, so thanks again. One thing I am struggling with is customizing the conversation and message objects a bit. Seems difficult to access the message model in order to customize it. For instance, if I want to add a few more potential fields to it, perhaps (i) a field marking the message as important or not, or (ii) a category label the user can add to a message. If it were a normal object, I could just add these items to the database table for the message (ie, t.boolean :important and/or t.string :category or something like that, and run a migration), and/or work with the message.rb file. But I do not seem to be able to access the message table or rb file. Are you aware of how to customize messages and conversations?