I'd suggest, for starters, doing a quick search to see how many of us have dealt with the issues of the requirements. You did hit on something important in considering the problem - that you're not allowed to rewrite/change the FFMS, as the client's told you explicitly not to.
You do need it, though - there has to be a provision for being able to allow flights using frequent flier miles. (It's in the Pay for Itinerary usecase.) If you think about it, there are ways to get around the "do not alter" requirement for FFMS - specifically, it doesn't say that you can't access the FFMS database, but rather that they don't want you rewriting it. So consider how you might allow for that in your design, if you choose to go that route.
And just for safety of consideration - it's not the only solution out there. Doing a search myself, I see others have tackled it entirely differently, so it's something you might think about.
[ October 19, 2005: Message edited by: Theodore Casser ]