If your computer case is large enough for a modern GPU Card and the computer has a strong enough PSU to run the GPU card then yes you can add a GPU card to your computer.
What normally happens is the moment you install the GPU card it will become the "Main Display" adapter for your computer unless you have in BIOS/UEFI set for specifically to use the IGP (APU Graphics). Normally that setting is set to "Auto". This means when it sees a GPU card installed it will automatically switch to the GPU card as it main graphics output.
This website lists the Minimum PSU wattage you would need to run your computer with different types of GPU cards installed. You can use this as a reference if you decide to add a GPU card: PSU REQUIREMENTS - RealHardTechX