How to add a missing instrument to an existing score

Friends,
HELP! I play piano in a mixed informal jazz/pop ensemble that includes alto sax, tenor sax, trumpet, flute, guitar, drums, and French Horn. We like the MuseScore scores for mixed ensembles (shout out to Jim Baker), but they seldom include a part for French Horn. Is there a way to transpose another part (say tenor sax or trombone) and insert it in the score for our French Horn player? Other ideas?