Adding more to the existing is fine as long as the new RAM is at least as good as the existing in terms of timings. The board will run all the RAM to the specs of the least.

If you were overclocking the machine then the best option would be to keep the number of sticks to a minimum and therefore replace what you have with higher denominations. For your purpose, adding to what you have should be fine as long as the board supports the mix. Obviously, keeping what you have will limit the max you can get.