In 9 you can do that with a point-and-click Transform template:
1. Open the table for your drawing.
2. Use the Schema dialog to add a text field. I'll name that field "WKT Text".
3. Switch to the Transform pane and choose WKT Text as the target.
4. Click the Compose WKT template.
Your table probably has only one Geom field in it, so that field will be loaded into the Geom box by default. You'll see the preview of how that is converted into WKT in the WKT Text field.
5. Press Update Field
See the Compose WKT template in the Transform Templates - Text topic.
There is a step by step illustrated example of an alternate way to do this in the Transform Templates - Drawings topic for the Copy template.