Is it possible to use keyboard shortcuts for CEP panel's function when the panel is out of focus?

I'm not sure what exactly you want to achieve but here's a suggestion: You could create a jsx file (or multiple ones) in AE's "Scripts" folder (not the "ScriptUI Panels" folder) which you can assign shortcuts to in AE's keyboard shortcuts dialog. (You can also add the shortcuts automatically by writing to AE's keyboard shortcuts file.) Now, I don't know about CEP panels but with a ScriptUI panel, you could call the panel from within your jsx file in the "Scripts" folder and hand over a global variable to tell the panel which function to execute. Or depending on what you want to do, just execute the function from the jsx file in the "Scripts" folder.

There's also a way to assign multiple shortcuts to a single jsx file while still being able to execute multiple functions. But it's complicated. See my explanation here:
https://creativecow.net/forums/thread/limit-to-after-effect-script-hotkeys/

As Mylennium says, there is no good, clean way to do that. I think the tool Excalibur  does this with a dirty hack - by running a process which monitors keystrokes in the background or something like that. Besides the security implications (nobody wants a tool to monitor everything that is typed) it causes limitations like the keyboard shortcut also being fired when a dialog is open.

I don't think it's in any way possible as the panel's event listener would block a lot of other functions and trigger a ton of errors, after which the script would stop and you'd have to relaunch it.

Mylenium