The only way you can do it, is with a proxy object. Create an object (cube or sphere, doesn't much matter) with your player model as its parent. Set its alpha to 0, then perform collisions on that rather than the player.
You may need to reposition the proxy object each frame by hiding it, repositioning, then showing (hiding disables collisions).
You may need to reposition the proxy object each frame by hiding it, repositioning, then showing (hiding disables collisions).