can you combine a regular return filter if you have a electrostatic hvac filter system
Yes standard fiber filters can be added, I suggest adding them at the return point if possible this keeps the heavy dust particles out of the duct system. Having a granddaughter that has severe breathing problems I can tell you keeping the electrostatic filter clean is your best bet. Adding fiber filters for large dust particles is the next best thing.
Adding filters that are very fine are actually counter productive as they slow the air down and reduce the exchanges through the filter system. Also systems are designed for a flow and excessive obstruction can cause shorter system life and even trigger safety components causing the furnace to shut down or cooling system to ice up.
The last few times where my area was choked with smoke from Forrest fires my grand daughter was not hospitalized as has happened in the past.
The electrostatic elements did need to be cleaned at 2 weeks where it is normally a monthly thing and there are the cheap fiberglass filters at the entrance(s) to the system plus the electrostatic.