Can I add more hardener to epoxy resin to speed up the cure time?

Adding more hardener will not make mixed epoxy cure faster because doing so changes the mix ratio, resulting in an improper cure.

To make the epoxy cure faster, you can raise the ambient temperature using a heat lamp or space heater. Colder temperatures slow curing; warmer temperatures speed up the curing process.

It's actually the hardener, that determines the cure speed, not the resin. If you want a faster cure, try using a fast speed hardener. Traditional epoxy resin systems such as West System 105 and TotalBoat 5:1 Epoxy Resin have a fast cure speed hardener: West System 205 Fast Hardener and TotalBoat 5:1 Fast Epoxy Hardener.

2:1 epoxy resin systems (MAS 2:1 Epoxy and TotalBoat High Performance Epoxy system) have three hardener speeds: slow, medium, and fast. You can combine the different hardener speeds to get the desired cure time and pot life for your project. Remember, the exact ratio is always required for a proper cure.