There's a keybind under targeting called "focus target modifier" and works exaclty as you describe. Bind the key, and then hold the key along with your taunt, attack, whatever. I believe you can't bind normal modifier keys like shift, ctrl, alt, etc. so you have to use something else. I use it bound to a mouse button on my Cyborg mouse. It's a little finicky sometimes, though. If you have someone as your main target and are attacking them, and begin holding your focus modifier and attacking DURING the GCD of your previous attack, it won't use your focus target it'll use your main target, so you have to let the GCD finish before pressing the key to attack your focus target. Not a huge deal, but if you're in the bad habit of spamming your keys like I am, it's a pain.