Yes, you can :D
Assuming that you are referring to the first generation Airport Express because the second generation comes with a cable.
The AC Plug Adapter from the Airport Express and the one from the MacBook's charger are the same. They are colloquially known as the Duck Head. Apple sells the World Travel Adapter Kit which all use the same pin. Additionally, you can try using any old iPad charger's duckhead, which goes unnoticed most of the time.
You shouldn't have to worry about the voltage because all that the duck head does is redirect the electricity directly to the charger without modifying the voltage. Unless your laptop is really old (which you've mentioned that it isn't), it probably has a built-in dual voltage. This means that your MacBook will function on the United States' 110-volt system, Europe's 220-volt system, and Australia's system, which can sometimes go up to 240 volts.