I don't see why that should be a problem. You can add pages to a 48-page passport as readily as to the 24-page kind - as I expect to ask them to do well before my new one runs out. The only legal requirement is that each set of 24 new pages be taped to an original page on both sides, not to another expansion set. From a practical point of view you can't get very many in, but unless you spend every week of the year going in and out of countries with full-page visas, you should be able to get enough.