Oscar,
I think you are mistaken about not being able to control the arranger from an external midi keyboard. I have a Yamaha PSRS910 being used as the controller into the BK5 keyboard midi input and it works fine. From the factory the "NTA" parameter in the "midi" section is sent to "off", but when you switch it to "on" you can have the keyboard of the Yamaha play and change chords on the BK5 just like you are playing on the BK5 itself.