could I just add ballast in the back seats then?
Be careful with this. Essentially yes you could add weight to the rear seats, but.
With lots of weight in the front seats, a lot of weight will be needed in the rear seat.
If you have the option of moving the front passenger, this might be the easiest thing to do. However if the front passenger is an examiner or instructor you'd be brave to tell them to move rearward!
To achieve the greatest effect in view of your problem, it would require about 35lbs further aft in the baggage compartment as opposed to roughly 130lbs minimum in the rear seat due to the shorter balance arm.
Hope this helps, and take Gomraths advice and work it over with your instructor.