You mean you want a separate collision box (or mask) for your player sprite? Yes, you can do that, and it's pretty easy. Create your box, (or in your case, a circle) and apply the movement you need to it. All of your controls will move that object. Then do this:
[code:2edfkbxh] +Always -> playerSprite: Set position to object hitbox.Sprite [/code:2edfkbxh]
Make your hitbox invisible and nobody will ever even know it's there.