I am creating a plugin to send SMS, and am currently dispatching a right hand sidebar to contain the front-end, but was wondering about integrating it into the left hand sidebar, with a section for sms conversations and channels for each conversation. Is there a way to create channels, or interact with the left hand sidebar to make custom components that will look like default channel conversations with custom methods and functionality?
Yes, I suggest you read the Mattermost API for more on this and other calls you may need to do.
I do agree with @MartinPJones, it seems that your best bet with this would be to create a bot with API and webhook(s) that would allow it to read and write information from the server, as well as to post messages in a specific channel, and create its own channels, etc.