There are several programs available that attempt to do this, with varying results. There was a function in XG works for this, but the results were rather poor when I tried it.
I agree with Roel in that it works best when done using convert monotimbral tracks, but none of the programs I tried a while back were very accurate even when I tried converting one instrument track at a time. Perhaps someone has gotten it nailed down a little better now, but I haven't seen anything that indicates this.
AJ
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AJ