Hello @RMAK,

It does not appear that OpenStudio has this capability yet. However, you can use the final .idf created by OpenStudio and model rainwater harvesting in EnergyPlus. It is a multiple-step process of creating a WaterUse:RainCollector component that references a surface in the model from which rainwater runoff will be collected, a WaterUse:Storage component that will model the tank where the rainwater will be stored and references the rain collector surface, a Schedule component for expected water temperature within the tank referenced by the water storage component, another Schedule component for the annual precipitation schedule (which can be found in the C:\EnergyPlusV8-0-0\DataSets\PrecipitationSchedulesUSA.idf file for your building model's location), and a Site:Precipitationcomponent that references the annual precipitation schedule and expected total annual precipitation (also in the same data set file). If you search for any of these components in the E+ documentation (or at [http://bigladdersoftware.com/epx/docs...]), you can find more info.

I have posted a question about rainwater harvesting and graywater capture on Unmet Hours, titled "Rainwater and graywater in EnergyPlus". Nobody has posted an answer, but you may find it of interest as well. Hope this all helps.