If you also want to have the data available in the original units then you should create a synthetic variable with a unit conversion calculation applied. See the article here for an example.
The simplest conversion is by using the scaling function. Select the correct device and variable and you will see something like this:
Enter values into Slope (gain) and offset. So in this case to convert from mph to metres per second you would enter a slope of 0.44704.
Do not forget to change the units of that variable to the new ones!!