Here is Roland Support's Suggestions.

Roland Product Support (Roland Corporation U.S.)
Oct 28 03:38 pm (PDT)
Hello Deane,
We are not familiar with the Ketron unit, but it looks like a small sound module, correct? If so, do you want the styles in the BK-9 to drive / play sounds in the SD-2? Or are you more interested in playing the SD-2 sounds from the keyboard of the BK-9?
If both cases, you will need to use the MIDI settings in the BK-9 to accomplish this.
To set up the sounds to be played from the keyboard of the BK-9:
1. Press MENU.
2. Cursor to "MIDI" and press Enter.
3. Cursor to "Edit Tone Parts" and press Enter.
You can see here that the four keyboard parts (Upper 1, Upper 2, Lower, M. Bass) have several settings - you need to confirm that the Tx (transmit) switch is set to "On" for each and also check the MIDI Channels. By default, the channels are:
Upper 1 = MIDI channel 4
Upper 2 = MIDI channel 6
Lower = MIDI channel 11
M. Bass = MIDI channel 12
You may need to change these to match up with specific parts on the SD-2 if it does not have the same type of configuration.

To set up the sounds to be played by the Styles of the BK-9:
1. Press MENU.
2. Cursor to "MIDI" and press Enter.
3. Cursor to "Edit Rhythm Parts" and press Enter.
You can see here that the eight Style parts (ADrum, ABass, Acc 1 - 6) have several settings - you need to confirm that the Tx (transmit) switch is set to "On" for each and also check the MIDI Channels. By default, the channels are:
ADrum = MIDI channel 10
ABass = MIDI channel 2
Acc 1 = MIDI channel 1
Acc 2 = MIDI channel 3
Acc 3 = MIDI channel 5
Acc 4 = MIDI channel 7
Acc 5 = MIDI channel 8
Acc 6 = MIDI channel 9
You may need to change these to match up with specific parts on the SD-2 if it does not have the same type of configuration.

These settings should allow you to trigger an external MIDI Sound Module (such as the SD-2) with the BK-9.
Best Regards,
Product Support
Roland Corp. U.S.