Is it possible to save a file from cs5 to cs2?
Yes, but only if you have CS4. Be aware that it opens up a can of worms of potential problems but should be OK for less complex documents in one off cases.
To do it export an idml from CS5, open in CS4 and export a inx file. Then go here and follow the instructions, or just do the same thing in CS3 if you have that.
Yup, you're a bit stuck. As I said it's not something you want to make a habit of anyway.
Useful tip for going from CS2 to CS5 if you're ever having any problems though, open the file in CS2, export an inx and open that in CS5, works better.