Well, I don't know, I thought you were talking about ailerons, but flaps are different matter. You can keep the single servo aileron set up and still add flaps. You would still want to mount the servos in the wings. I presume you are now talking about actually adding flaps and not a "flaperon" set up. I would not recommend "flaperons", because you would have still have to use a separate left and right servo and the performance is not as good as it would be by adding flaps in addition to the ailerons. For almost the same amount of work,, it would be much better to leave the ailerons alone, add the flaps, and mount the flap servos in the wings.