Macs have a feature where long-pressing a key brings up a list of accent characters, which looks like this:

I’d like to know if it is possible to change this list of suggestions.

In particular I type a lot of mathematical equations and would like to add the common Greek letters and other commonly-used unicode symbols here.

I have no idea where to start with solving this. I’ve searched around and found questions like this one about enabling/disabling the feature, but nothing about changing the list.

Thanks!