Same thing here. Went to the fire department to have carseats installed and the firemen told me not to use anything in the carseat that didnt come with the carseat.
The reason is because the carseat wasn't crash tested with the body support things, seat belt pads, etc. They told me to use rolled up recieving blankets to support baby's head.
Also-keep the handle pulled and locked back when the carseat is in the car. They told me this is because if in a crash the carseat is designed to be pushed forth. Baby's head will come up and smash into the handle bars.
One more thing-No toys on the handle. It makes sense. If your driving 75 on the highway, get into an accident do you want to have that thing come smacking you in the face?