I'm a designbuilder user, not BEopt or OpenStudio. This is how I would set it up:
Create a mixed water loop by using a plant loop with the load distribution scheme set to 1-sequential, and the plant loop demand calculation scheme set to 2-DualSetPointDeadband. Set the main plant loop setpoint manager to type 17-dual setpoint. I use high setpoint = 24C and low setpoint = 7C.
Then connect your components in series with appropriate setpoint managers. Connect the ground heat exchanger (GHE) first and use the same setpoint manager as the main plant loop. Then connect the boiler, and cooling tower, all with the same dual setpoint manager.
Set the plant equipment operation such that the GHE operation type is uncontrolled, the boiler is heating load, and cooling tower is cooling load.
In theory, as the load increases past the capacity of the GHE, the boiler or cooling tower will pick up the load.
I hope this helps.
Greg