Multiple Occupancy Sensors to control hot water return pump
I am assuming the sensors in the bathrooms are low voltage, say 24V, and rated OK for parallel operation.  Also assuming that the relay that runs the pump is the same low voltage.  Generally the sensors will have three wires, positive in, negative / common in, and switched out.  Let's say it's black, white, and red.  

In the basement, you'd connect the all the blacks from the occupancy sensors to 24V+, all the whites to common, and all the reds to run them in parallel.  

Then to run in series with the temperature sensor, you'd tie the reds to the black of the temperature sensor, the white of the temperature sensor to the other whites, and the red of the temperature sensor to the coil / primaray + terminal of the relay that runs the pump.  

The white of the coil of the relay that runs the pump would tie to all the other whites.  The secondary / contacts of the relay would switch power form the pump.