The general advice for RAM is to get it in a full kit and replace the old. The reason being that a full kit is tested for compatibility and voltage tolerances at the factory.
It is possible to mix and match RAM, but may require manual adjusting to avoid some issues. I'm personally of the mind if a person is comfortable with manually adjusting their RAM timings and voltages, then mixing and matching RAM is fine. If a person isn't comfortable with that then fully replacing a RAM kit would avoid the potential issues. I'm personally of the latter category.
But yes, it is feasible and an option to keep your current RAM and allow for more to be spent elsewhere.