attach switch/push sensors directly to keyboard keys

I need sensors that will attach to a keybord and work in stead of key-press/inputs. F.ex. if someone steps on the sensor it would be equal to pressing the "1" key, another sensor = pressing the "a" key and so forth. Is this possible -- can it be done by simply taking the keybord apart ... extending it with long wires.
Not good at describing this but still hope that you can help.

A possible solution: connect an optocoupler parallel with desired button pins. It can work only with right polarity, when no action, change the emitter and collector. The LED-side can work with extreme long cables.