To add pages to your passport, all you need to do is complete Form DS-4048: Application for Additional Visa Pages and mail it in with your current, valid U.S. passport to one of the following addresses. Click here if you need to expedite your passport.
If using USPS:
National Passport Processing
P.O. Box 13408
Philadelphia, PA 19101-3408
If using other deliver service:
National Passport Processing
Attention: Department 3408
1617 Brett Road
New Castle DE 19720
If your passport is only a few months from expiring, you may opt for renewing it instead of adding pages.
There is a new fee of $82 for this service. If you need to get the pages added quickly, then, you'll have to expedite it and pay an additional $60.
The U.S. Department of State Passport Service offers an expediting service. The turnaround time is 2 weeks or more.
If you need your passport sooner, you can set up an appointment at one of the regional Passport Agencies. If you cannot travel to one of these locations, you will have to contract a passport expediting service.
If you need to get your passport fast and want to apply online to have it expedited, visit Rush My Passport.