This would be not possible out of the box and could be done with a customisation. Calls for an Enhancement Request !!.
It sounds like you are trying to simulate a transition comment like the comment that users enter while making a transition.
I'm not sure it is possible to set the comment without API programming.
Here's a couple threads about it:
Hope this helps.
This technique works if you want to send the notification to a group or a user that is known in advance. It will not allow you to send the automatic comment to the file Creator, Last Modifier or to a specific user selected via checkbox (Dynamic) such as can be done with a standard transition comment.
If your application can tolerate these limitations, you can use Tony's suggestion, otherwise API programming would likely be required.