It sounds like you're describing a way to create and modify messages in the HubSpot conversations inbox.
At this time, the conversations inbox is not accessible via API.
I found this feature request which describes similar use cases, however. I recommend upvoting the idea and commenting with your specific use case to let our team know you are looking to write and retrieve email messages to the inbox.
Thank you for upvoting and commenting on that feature request.
Unfortunately, I don't have an API-based solution for you.
Do you work with any external email systems which can automatically send or forward emails? You could use such a system to send emails to the account connected to your Hubspot inbox so that the messages appear.