I suppose you know what you are doing, exporting a project at 24 fps when it is made from 25 fps footage? So I'm not gonna comment on that...
FCPX only exports to the settings your project is in. And from what I've seen, you can not change the frame rate of a project that already contains footage. But you can go with a simple workaround: Mark everything in your project and cut it (Command-X). Go to the project settings (Command-J) and click on the tiny wrench at the bottom right of your inspector. You can set several frame rates, including 24p. Now paste everything back in (Command-V) and it should stay in 24p.
However, then your whole project will be in that frame rate and you will lose quality with all footage that has been shot at a different rate.
Edit in one frame rate and then export to another is not possible to my knowledge. You will need to spend the 50 bucks on Compressor to do that. FCPX is not that powerful on export settings, because it relies on Compressor for more advanced users.