It is easy enough to write text into a message using VBA, but inserting Autotext is an altogether different problem. Frankly I have not come across a solution to this issue.
If I was faced with this issue, I would do it in one of two ways.
1. Create separate Outlook templates as required. You can undoubtedly use your autotexts to create messages which you can then save as templates. Then create messages using the appropriate templates as selected from your dialog.
Set olItem = CreateItemFromTemplate("C:\Path\TemplateName.oft")
2. I would create the messages as Word documents, using your autotexts which are much simpler to insert in Word documents, then send the documents as messages to Outlook.