If you attach stuff to it, it may not be a certified safe helmet any more.

But so what? Then it's just a hat. There's nothing that says you have to wear a helmet and can't wear a hat.

You can't SELL something in the US that claims to be a bicycle helmet unless it meets CPSC standards, but you can wear whatever the hell you want as long as you're not violating vehicular code (like having a flashing blue light on your helmet or something).