Yes, you can install an electric element in the plenum. Plenum heaters, as they are called, are available on the market.
Most of the time, the plenum heater is installed next to the heat pump’s cooling/heating coil.
If you pay Hydro-Québec’s Rate DT, the electric plenum heater and heat pump stop working when the outdoor temperature gets down to -12°C (or 15°C in some regions) and the oil-fired forced-air heating system takes over. But if you’re just thinking about replacing your oil heating system, we recommend installing a new electric forced-air heater.
A heating specialist will be best placed to advise you on the subject.