My understanding is that the iPad app is only for editing performances (registrations.) The BK-9's built-in Style Composer is a little clunky but workable. I know there is a menu command that allows you to delete a whole range of measures. It's also possible to remove content from, or mute individual tracks. Take your time, experiment, and don't forget that there are actually three sub-variations within each style section (used for Major, Minor, and Seventh chords.)

If you really, really prefer to work on a PC sequencer... With the BK-7m, Roland released a software called "Style Converter 4." This allows you to import a MIDI in a prescribed format, and re-encode it as a Roland style. That would be about your only option. You would still have to add Alteration Mode messages in the BK-9 because the Style Converter and some other Roland arrangers circa 2009-2010 lacked that feature.

Good luck and please share what you learn!