Hi - I know nothing about keyboards, but if you've had the cover off, maybe you could see if it had a stereo type TRS jack? If so, it is porbably wired as an insert commonly seen on mixers. One wire is ground, one wire is a 'send', and the other is for 'return'.
Replace the input jack on your pedal with a TRS jack also. Disconnect the output and rewire it to the new input jack. There are only two connections, and one is ground. The second wire would go to either of the remaining two terminals, the 'T' or the 'R'. (The old input wire taking the remaining terminal.)
If it does not work, simply reverse the T and R connections. It's a 50-50 shot.