Can I use a secondary thermostat as a timer for my air conditioning system?
Yes, it is just that easy.

You will need a timer with a 'dry contact' or relay output.

If you take power from the pump, remember it is (likely) 240 volts.

Removing power from the pump will not remove power from the emergency heat strips, or from the fan.

Removing power from the pump may, or may not, turn off the reserving valve (Heat mode / Cooling mode) in the pump.