If it functions as an electrical storage mechanism with a particular capacity and a particular round-trip efficiency, then the changes we made to HOMER’s battery module should allow you to model pumped hydro.
When you create a battery you can choose whether to specify the capacity with a capacity-versus-current curve, which would be appropriate for a lead-acid battery, or just a fixed value is Ah, which would be appropriate for a pumped hydro installation. Also, you can choose to specify the lifetime in terms of years, kWh throughput, or both. For a pumped hydro installation I suspect you would just want to specify the lifetime in years. You can also choose to specify the maximum charge and discharge current separately.
The ‘battery’ must still connect to the DC bus, which is probably not what you want with a pumped hydro installation, but provided that nothing else connects to the DC bus, you can work around that by specifying a huge, free, and 100% efficient converter.