If I have some video which I base my sequence on, and want to keep its audio, but also add music here and there... and the music I add happens to have a different "frame rate" (Hz)... how do I deal with that? Is it possible to convert MP3/whatever audio files to whatever a given sequence uses within Pr?
When you Import your Audio, it should be Conformed to 48KHz 32-bit floating-point for editing, regardless of its original Sample-Rate (within reason). Be sure to let all Conforming complete 100%.
As for MP3, that might work fine, but if not, you can use Adobe Audition to convert that to PCM/WAV @ 48KHz 16-bit for Import.