Is it possible to connect custom sensors to Tinkerkit?

While Tinkerkit has a variety of sensors, some seem to be missing (such as PIR). Is it difficult to connect & program additional sensors for Tinkerkit? Also, do you know where it would be possible to buy Tinkerkit compatible sensors?

Any sensor compatible with arduino should have no problem connecting to Tinkerkit. It's just easy with Tinkerkit sensors you just plug them in. With other sensors, you need some jumper wires.