If you can get to a US Embassy or Consulate during your travels and can leave your passport there overnight, you can have the extra pages added. (To be clear, it used to be 'while you wait' and free of charge, but now it's overnight - thus a Friday drop off means a Monday pickup - and $82.00.) BTW if you have at most one insert, have them add two inserts, same price as one. As info, I got two inserts put into my passport book at the US Embassy in Singapore back in December.

It seems to me it would be faster if you did it at an embassy than to go back home and send your passport to the Passport Office.