no u cant. that is one dissappointing thing about halo 3's forge. but in halo reach, u can make things float in the air. when u are in forge mode and are holding onto an object, press X to show object options then go to physics. then u can switch between phased (can float in air and go through walls and everything), normal (dont float in the air and cant go through walls - just drops to the ground), and fixed (where it can float in air but cant go through walls). hope this answers yuor question