Flaps are usually used as an airbrake or to make the stall speed slower. On a trainer, the stall speed is already so slow that with flaps you would almost certainly need to cut the engine to get it to quit flying.

As far as adverse yaw.... that is what trainers do. They have lots of dihedral. Your best bet is to take advantage of this problem with the airframe and learn to use the rudder if you haven't already. Try flying your airplane without using your ailerons, use your rudder to turn. That will add a new challenge.

Once you have learned all you can from the trainer you can either do as bruce suggests and remove the dihedral from the wing or convert it to a tail dragger, but I wouldn't add flaps, it's pretty pointless IMHO.

Keep in mind that there is a lot you can learn from a trainer, so keep at it until you are using the rudder as much as (if not more than) your ailerons and inverted figure 8s are no big deal, then get something a little more sporty.