The bank/program change midi message is the crucial one, and as Diki explained, a lot, but not all of the values required for the Ketron sounds can be sent by the BK-9 (by finding the BK-9 sound in the tone list with the respective value).
Let's have an example: I want to play the Ketron sound 'Mandolin' in Voice Bank B, No.106. In order to call up this sound from the BK-9, you have to find a BK-9 sound which sends out the midi PC message 106 and bank (CC00) message 1. In the BK-9 tone list I found sound no. 1535, "Pipa /4", which has the value CC00=1, PC=106. So selecting Pipa /4 on BK-9 should trigger the Mandolin sound on Ketron SD 2. Of course, you would have to mute the BK-9 'Pipa /4' sound then to hear the Ketron. Well, that's a way to use the Ketron, but it remains inconvenient...