I know that this is a late response to your original post but I didn't see if your query was answered so I'll post my experience..
I've been buying updates to BB since the mid 90's and have accumulated hundreds of styles.
I've been able to import these into Songsmith successfully by copying the .sty files from my BB folder into the "styles" folder under the Songsmith installation folder. I now have access to all my Band-in-a-Box styles in Songsmith.
Hope this is useful.