Welcome to the forum @Jacky_Wong. I’ll have a think about this in general but in the meantime there is something you can do right away: you could use a Snippet to define your own DeepL extension.
Here’s how:
First, in DeepL, set a different shortcut — in the example below, ⌃⌥⌘D:
Then go to a text editor (TextEdit is fine), type out something like the following, and select it with PopClip:
name: DeepL
icon: circle D
key combo: control option command D
Of course, you can just select the text above, if you aren’t changing the key combo 
Here is an example of me doing this:
You’ll need to change the “key combo” if you use a different shortcut. You can also change the name of the extension and you can change the icon.
Here is more information about Extension Snippets.