I've never added pages when in the US, but have done it a few times over the years at different US consulates overseas (I think twice in Madrid and once in Phnom Penh). IIRC, there is no charge when having pages added at a US consulate, and it's done while you wait -- I don't think I ever waited more than an hour at the most; adding in time for and going through security, you should be able to get the whole thing done in 2-4 hours.

Why not just build in a little extra time (say, a half day) on your next overseas trip? Your employer should be happy to accommodate you, as it will certainly be faster than sending your passport to Philadelphia and waiting for them to turn it around.

As for the page numbering, my current extension is numbered with letters, rather than numbers. So I have pages 1-11 (or whatever) and then A through W (or whatever), and then 12 through 24.

- Michael