Ah I see. Electrically what you want to do will work, however there are several issues.
Firstly, the JB7 seems to be quite a small amplifier so if you were to give one of the channels (obviously you can only plug the sub in to one of them and not both) extra load by way of the subwoofer, it might be too much for it to handle. Also even if it could handle the load, it would make the channel you plug it in to notably quieter since the sub and one of the MR1's would be essentially fighting over the power.
Secondly and more significantly, a subwoofer channel is designed to only be for low frequencies, and they tend to sound very bad when you give them higher than what they are designed for. Given that the SC-BT230 also amplifys the sub channel, it would make sense to me that the low frequency filtering is done inside it and not by the subwoofer itself - though this is just a guess. If this is the case, the JB7 will give the sub a full range signal and it will probably sound really bad, and not at all what you were hoping for.
I hate to say it, but I think a seperate sub might be the best way for you. I've looked at the manual and the SC-BT230 doesn't have an aux output, so it's not like you can get a small mono amp and a few wires to hook it up that way either.