Mouse axis movement syntax for remapping?

I don't know if this is possible or what syntax to use.

I want to basically map (Middle Mouse button + Mouse going down on Y axis) to Down Arrow.

I've been playing a MOBA and they are in beta and don't have rebinding keys enabled so I figured I could use autoit to do it myself.  I'm trying to remap middle mouse + all 4 mouse directions to the 4 arrow keys.