I'd love something like this too. So bloody annoying to have to keep flicking the cursor all the way up to that tiny little focus-fire button.
I always go into my Keybinds and set Focus Fire to the ` key. It's right next to my attack buttons (1, 2, 3), which makes it really easy to hit. Plus setting it up this way, instead of on the same key as switch target, allows me to target enemies that are Exposed without interrupting my crew's focus on their target.
How do you bind things to keys like print screen or page down? Does the /bind command work with them?
You have to know the program's name for the key. You can learn this by going to Options>Keybinds and temporarily setting a key for a command - use the 'Select special key' button to see the list of names.
(BTW I believe the print screen key is called PrtScrn, but I could be wrong.)