Can I add extra padding or pillows to this car seat?

No, do not attach additional padding, toys, or other devices not made by Evenflo or described in these instructions. Items not tested with the child restraint could injure the child, and additional pads or pillows may interfere with the function of the harness system or cause breathing problems if the child's head falls forward, increasing the risk of serious injury or death. Only the optional Infant Head Support and Body Pillow are permitted for rear-facing use. (Page 14)