At first, thanks for the quick answer...
Unfortunately, i do not know how to work with the file "Band in a box....xml" (which I have found, of course) and am afraid to do something wrong (!).
As explained, I have already added other instruments which apear in Sforzando at the same level as "Band in a Box sounds" (like "free sounds" or "TableWarp2").
When I open (under "Programs" the Plogue folder, (instead of the PG Music one), I have all these folders: Aria, Free Sounds, TableWarp2. Unfortunately, just adding a new folder Virtual instruments with all the instruments under-folders does not do the trick... Aaargghh!