Slap Bass instrument

I imported a mxml file from flat.io, which had a slap bass instrument in the score. It clearly imported the slap bass instrument onto musescore, but I can't add another if I try (lots of people know that slap bass isn't an available instrument in musescore). I was wondering how the slap bass instrument got onto the score, and if/how I can add another one/add one on another score.