OpenStudio uses EnergyPlus measures to add tariffs to a model, so you can add any tariff structure that EnergyPlus supports. There is a Tariff Selection Generic Measure on BCL BCL, that lets you add any IDF files to a "resources" folder to use as tariffs. It includes a number of example tariff tariffs structures that may serve as a starting point for you.

Additionally here is a link to the UtilityCost:Tariff documentation for EnergyPlus.