You don't just add it. You'll need a Windows 10 installation DVD/USB, so you can install it, or update from an eligible windows version (7,8,8.1) to 10 if you are already running one.
When you install Windows using BootCamp, or one of the virtualization options above, you get Windows just like what you get on a PC. Its not a version of windows, or some windows features. Its a full Windows installation. So you get everything that comes with that.
With that said, Bob, is correct. Apple has not yet released Windows 10 drivers for bootcamp, so there are many things that still don't work correctly in the Windows 10 installation. You can try a Virtual Machine, but you will need a fully licensed Windows installer either way.
http://www.amazon.com/Microsoft-Windows-Pro-Flash-Drive/dp/B01019TDJ8/ref=sr_1_1 ?ie=UTF8&qid=1438357079&sr=8-1&keywords=…