Flaticon

Frankly, your question is not clear enough. So we will divide into these 2 directions:

- If you want to add icons in the available libraries of the theme, it is impossible (such as flaticon, awesome, themeify, ... ) Because it will overwrite the existing icons.
- If you want to add icons to new, independent, and unrelated libraries that already exist in themes, it is possible ( such as flaticon-2, flaticon-3, flaticon-n, ... )

This will be the structure to guide you on how to add:
- Copy the library of icons you want to add in Theme, Child theme.
- Redirect that library in back-end và Front-end.

(Note: To do this, you should have the knowledge about PHP, WordPress, Theme, ... ) then we can continue to guide you. If you know nothing, all guideline will be meaningless)

Regards,
Yame