I've got some BiaB .STY files, and as I noted above, looking inside them they do NOT appear to be, in any way, midi files/data, although there could well be some note data there.
I've just downloaded some Yamaha .STY files, and these look like a sort of midi file (.SMF), there are MHdr and MTrk blocks there, and data that could well be midi data (mostly) but there is other data there as well that may not be normal midi, or midi at all?
So, the file formats are NOT compatible, even remotely so.
I have the manual for my BiaB, from back in 1991, and there is full info on using the process to create/amend/use the styles, I don't see any info regarding the format of a STY file. I assume that you MUST use the special software. I assume that the 'midi' type of STY within the newer/current versions of BiaB are similar.
There is a lot of info on the web regarding the Yamaha styles, and a lot of files you can download (usually as 'collections' comprising a number, usually substantial, of different style files. There also seems to be software to create/amend etc the files.
Note that Yamaha produces a range of 'Arranger' keyboards that can use the styles, and the files for different keyboards, and different levels of keyboards, may not be fully compatible, although it may not be a major task to cross modify. The bigger the difference between two keyboards, the more complex will be the changes required?