Best way would be a hardware conversion- terranex at a post house. But that's expensive. Next best would be a dedicated plugin such as nattress.com's Plugin. But that's not free either. With FCP alone you can send to Compressor, set for the PAL settings and frame rate and jack up the quality levels on the Frame Controls. Will take a while but this is about as nice as can be with Final Cut Studio alone.
If the project is in 24p, would it be possible to just edit/encode your final output at 24p and then use Cinema Tools to conform to 25p/50i? I mean there'd be a slight speedup in the footage, but at least the process wouldn't be time consuming or expensive.