I bought the Iphone5 in HK, can I use the same USB charger in US with different voltage?

Yes ,the power pack should show 110-240v 50/60hz

A world wide useability device ,expect it will require a pin converter

All you need is a plug pin adapter...check for that in HK as unless it uses the flat-US style plug pins conversion to US will be dificult to find here.

Good luck, Nick. With all of the electronics stuff available in HK that should be really easy to find.