Chat SDK uses ChatKit for the message view so I would recommend that you look at their documentation before trying to add custom message types to Chat SDK. Chat SDK really just exposes the ChatKit custom message API.
Override Text Message Display
In this example, we will override the outcoming text message so that we can append "Hello World!" to every text message.
To do this, the first step is to make a new class that extends the OutcomingTextMessageViewHolder.
This may seem complicated, if that's the case, I would recommend thoroughly reviewing the ChatKit documentation. This guide just explains how to access the ChatKit API from within Chat SDK. It doesn't go through the ChatSDK API in detail.