Great question! Computer software manipulation will need to be coming from computer compatible hardware. The hardware would need a USB connection built in, or something along those lines. You are unable to connect a 1/4" jack to a computer. The correct singnal needs to be going between the hardware and the software. 1/4" cables are for transmitting audio signal, while USB cables are for transmitting computer data.

It also depends on what you are trying to achieve. You were trying to plug your expression pedal into your keyboard, so are you trying to use with with keys, or do you also want to use it with a guitar? Does your midi controller have tone wheels? These are often used to manipulate your keyboard sounds. Or there may be an expresssion pedal which has a usb plug? Otherwise, simply plug your rig into an amp and record the amp with a mic! Classic.

Good luck!