I recently had extra pages put in a the LA Passport Office. I paid for the expedited service -- $60 I think but it could have been $40. I made an appointment (very important!) for Thursday and the passport was ready by Tuesday, just went back to pick it up. A year or so ago (before the big rush got going) a friend sent his off to have pages added and it took well over a month. May be worse now.
As was mentioned above, the inserted pages are lettered. They are the color and style of new passports and so don't match my 5 year old passport. When I used it the first time in DUS it got some close looks and a few questions, but no hassles.