If you subscribe to the IFR data package, so that you get access to the IFR approaches, then iFly already does what you're asking for in a different way. The approaches (most of them, anyway) include FAA-supplied altitude constraints for waypoints along the approach. iFly already has a "Vertical Speed to Targe" (VST) instrument that calculates your required descent rate (or ascent rate if your next waypoint is higher than your current altitude).
Load the approach, fly the approach, and use the existing VST instrument for guidance to achieve your target altitudes along the approach.
That said, I don't know why a VFR pilot would want to fly an IFR approach as an aid to a VFR landing. It's not consistent with the normal VFR landing pattern and puts the plane in potential conflict with IFR aircraft flying the approach.