Yes, you can open the door of a fridge or freezer during an equivent.
Guidelines from UK Power Networks state that in the event of a power cut or when power has been turned off, food is safe in your fridge for at least 4-6 hours with the door closed and a full freezer can last up to 24 hours.
A powerDOWN event usually lasts about an hour, it is safe to open your fridge door quickly to minimise the amount of warm air you let in. We would also suggest minimising the number of times that you open a fridge/freezer door during an the event.