[Baertram]

I know. The info provided in these threads should help you see what needs to be done and checked in that case, e.g. check addon descriptions, licensing, contact author, what time to wait for an aswer at least, etc.

We cannot provide a 100% answer here, it's up to each author and addon itsself (especially if any licensing is used).
Pleas read the linked information and if you got further detailed questions (e.g. describe what the addon uses as description/licensing, if author said anything about using his addon's cod/changing his addons, and what code in detail you want to copy) feel free to ask here.

If nothing is against reusing the code, or if it's even ZOs code and he only copied that, you can also do so by providing proper credit in your addon's description (and at best directly at the code lines via comments) to the original addon/author.