Yes, the BK-5 (and the BK-7m, and the BK-9) will play older Roland styles. The intent of the Style Converter software is to create styles on your PC by repackaging MIDI files into the Roland-prescribed format.

Since most styles from other brands' keyboards are already MIDI files, if you're really good with MIDI and you understand how the style data is packaged, it would be THEORETICALLY possible to convert another brand's style or MIDI file to a Roland-compatible style.

However, I know that the Style Converter software (at least version 3) lacked the ability to add certain proprietary control features to the style. For example, events like Alteration Mode messages would need to be inserted using the Rhythm Composer function on the BK-5 itself, or another late-model Roland arranger.

I hope Nigel doesn't mind me mentioning another forum, but you might be able to get more brand-specific information at:
HTTP:\\www.roland-arranger.com

My answers:
1. No intermediary step. Just load the old style and it should play.
2. The Style Converter software fills in a "template" which packages MIDI data so that your keyboard will understand it as a style (with fills and variations directly accessible using the buttons, etc.)
The idea isn't to convert older styles or other brands' styles (although you might be able to do that.) I think the idea was to convert MIDI song data to a style. It isn't well-developed or especially useful, IMO. My $.02, you might get more opinions / feedback on the other site i mentioned.