Hey there,
I'm not the host for this jam, but honestly don't see why you shoudn't be able. However the jam has a raiting system with the following criteria
Visual Impact - is the image impactful and interesting?
Creativity - Is the design creative and/or unique?
Technical Execution - Lighting, perspective, color, etc.
Correlation to Theme - Did the artist write to the theme, or go off the rails? This is largely to ensure original work.
Most likel having only sketches might not give you the best scores in some of the categories. But still worth a shot.