Coolant is "leaking" out of the radiator cap while I drive, but the level of fluid in the coolant reservoir doesn't seem to be decreasing at all. I really don't know how much is coming out of the radiator, and I don't know the level of fluid in the radiator. I may have over-filled the radiator's coolant reservoir before this started happening..

Can I top off the radiator with coolant even though the reservoir is already full? Any other tips? Thanks!

Yes you can top off the radiator when the engine has cooled and no pressure is in the system. Do not rely on the coolant bottle level as your reference of needing coolant. The coolant bottle can be full and the engine and radiator could be empty, especially if the radiator cap is faulty. Replace a leaking cap and make sure the radiator is full to the top. You may also want to test the recovery bottle hoses and pressure test the coolant system.