I do not understand your last sentence. SingleTarget and CircleGraphic are independent; you can set the threshold window to any size that you want and it can be either larger or smaller than the circle size. If you assign a CircleGraphic object as a target to SingleTarget, the threshold window is initialized to the size of the circle, but you can change it afterward as you want.
Currently, MultiTarget uses the same window size for all its targets, so what you described cannot be done with MultiTarget. You just have to use multiple SingleTarget around the circle. See the attached example.
By the way, I do not think this trick will help the animals learn to touch the central area. What they need is visual feedback of the touched location. It will be more effective if you just use multiple concentric circles of different colors and show the touched circle again during reward.