The message for this is very inconsistent. I've been told different things by different consular officers. The last time I had them added (at the US embassy in Zagreb), they said that this was the last set of pages I could have as it was my third. I have heard other FTers recount getting five and six sets of extra pages.

Regarding the insert: they are not sewn into the book. Rather, they are attached with a very strong cellophane tape. I would be astounded if there were another way to get them in, as I believe they only come in that single format.