Adding new instruments to sound fonts

You are going to need a SoundFont creator such as Vienna or Polyphone. You will also need audio samples (the more of one particular instrument the better) of each note and all that as well as articulations and all that. Then you are going to have to create a SoundFont for that one particular instrument or add it to the set you want to add it to. I forgot to mention, you will need to work on more complicated things such as balancing out the instrument and making sure each note is in tune. An instrument sample becomes annoying to use if one or two notes are obviously out of tune. If one note is louder than all the rest when the dynamic is consistent, then that also makes the instrument awful. Another thing you will also need to work on is looping the sound. Looping is necessary if you want the sound to be able to play over a long time. You will also need to make sure that the looped audio does not sound like a short sample being played over and over, because that will be awful, but rather a long consistent sample.

Keep one thing in mind. It is difficult to create a SoundFont. You also can not expect it to be used in the main SoundFont (the one that initially comes with the MuseScore download), it might be used should the developers deem it worthy enough, but in general, don't get your hopes up. Another thing, the more massive a SoundFont is, the more likely it will make MuseScore run slow.

I like that idea :)