I could be wrong, but based on everything I've seen in NCH software, it's unlikely MixPad has the capability you're looking for. That would require analyzing the frequencies and trying to determine all the notes that are being played and for how long, which is not easy, and then spitting out an attempt at approximating that in MIDI. There are some programs that can do this, but the quality varies greatly and none of them will be error-free, particularly if you're talking about a fully mixed recording of multiple instruments (extremely hard), vs. a single instrument (much less hard, but not easy) or something made up of only single notes (way less hard, but still unlikely to yield perfect results).