There isn't an event on the control that automatically produces a tapered side.
I would not use the frame mill on this part. Why not? Because you could use the frame mill to step down into the feature you are trying to produce, but it would require a fair bit of math because you would have to calculate the start point for each frame (the start point is the lower left corner, not the center). You would also have to calculate the length and width, which would become smaller in a linear fashion. The real problem would be that the sides of this feature would be steps and not smooth, even after all the calculations.
I think you would get better results by programming a profile of several moves in the YZ plane, then using the Repeat event to step along the X axis incrementally. A couple of hints: a.) the YZ profile should end at the same point it begins and b.) the step along the X axis must be an incremental move (there must be a "/" after the number). This type of programming assumes you're using a ball nose cutter and you won't be able to use any cutter compensation, since cutter comp on this control is only 2-dimensional.