Typically, in OneNote, on a Tablet PC, the user will configure what is drawn with the Pen, as being Text, Graphics, or Text with graphics.
After drawn, the user can circle what is drawn with the lasso, and if the graphic is configured as Text, or graphics with text, then the "convert to text" option is available and can be performed on the lassoed graphic.
The reason I mention that is it may be possible to use OneNote to convert scanned images to text, but if so, OneNote will need to "see" the scanned image as text capable, or mixed text and graphics.
I do not actually have OneNote installed, but I have done some advanced troubleshooting on it.
I have not tried to import a scanned document image and read it because all of my work has been oriented at making the Pen work correctly and some "localized" language issues.
I do know that even without a Tablet/Pen, you can use the mouse to write text and then you can use the same method to capture the text regardless of having a pen or not.