great project to make your own crash test!
Most non-Littlebits sensors need connections that are not standard available in the Littlebits world. I found an accellerometer from adafruit, the MMA8451 , see here:
Adafruit also has a great tutorial on how to use this sensor.
It can be connected to the Littlebits w6 Arduino bit, but it needs a lot of precise soldering. You need to connect wiring for VCC (Vin on the sensor), GND, SDA and SCL which all are present on the surface of the Arduino....
I did the same for another sensor, used in my marbles revisited project, with the same type of connections, the color sensor TCS34725, see this diagram for reference:
SDA is D2 and SCL is D3 on the surface of the Arduino.
Hope this helps,