Offsetting shape keys that start from -100 to now start from 0, for Unity import?

I am building a model in Blender which I import into Unity 5. It is set to smile at a value of +100 and frown at -100.

However Unity only allows for the value range of 0 to 100 and I can’t make my model frown.

Is there any way I could have Blender offset things so that 0 is a frown and 100 is a smile (which sounds kinda iffy…)?

I suppose I could use two shape keys for this but I was hoping for a minimal number of controls (and there will be lots if I have to use two for everything)

What would you all suggest?