Castlevania SotN uses almost exclusively streamed music so you'll have to be very familiar with the XA format, how the games make the disc drive play it and loop it. The music is interleaved so there's data from 4 songs at the same memory location, interleaved with eachother. You will probably not be able to insert a music tune without removing another of at least the same length.
It's a bad idea to convert from MP3 to XA. Both MP3 and XA are lossy compression format, that means they remove some composants in the sound for it to compress as much as possible. XA uses a fixed compression rate of 1:4, so it takes 4 times less data than the CD format, and uses ADPCM compression. MP3 on the other hand has multiple possible compression rates, you can find a MP3 that compress only like XA but it's likely you have one who compress much more, affecting the sound quality more. Taking that garbled data and compressing it again in XA will remove even more quality, and sound badly. It's much better to extract the sound data from the Saturn game by yourself - or even better - directly from the soundtrack disc who might if you're lucky not be compressed at all.