I suppose. You can play in the NA servers with an EU account, as long as your IP is a US IP.
Combat Arms NA and Combat Arms EU are the same game, but require different protocols to run correctly. You don't need an EU launcher to play Combat Arms EU; just download the direct version from the Nexon EU website. If you already have the US Combat Arms installed, try going into the game folder and click on the .exe files until you find a Nexon Europe login prompt. Just sign in with an EU account and you're off!