You can add new coolant to old coolant as long as the coolants are compatible and the same.
If you just need to top off coolant it's okay to add some new coolant in with the old coolant as long as the old coolant is still good and not contaminated.
Make sure you use the correct coolant as mixing different types of coolants could cause issues with the cooling system and even lead to corrosion and clogging or even gelling.
It's best to drain out the old coolant and put in fresh new coolant, but if the coolant level is down just a little bit then adding a bit of new coolant to the old coolant is not usually a problem.
When adding coolant start the engine with the radiator cap off or if your vehicle uses a coolant reservoir you can add the coolant to the reservoir and allow the engine to idle for a few minutes.
Watch the temperature gauge and ensure that it does not overheat.
The engine temperature may go up and then drop back down when the thermostat opens up.
If you have an owners manual for your vehicle the coolant type should be listed in the owners manual.
Or if not you can input your vehicles make and model and year into Google search and it will show you what type of coolant your specific vehicle takes.