You can add them to PG C:\Program Files\PG Music Inc\Band-in-a-Box Sounds/SFZ Library Band-in-a-Box Sounds.bank.xml
then they will show in the list in Sforzando.

http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=419866#Post419866
if you can't get it worked out let me know.
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