there's something i dont get. I can add a greeting for all NPC but the other top?cs wont show how can i add my quest topic globally to any npc in the game, without eclipsing the other conversation?

Click on a topic in the left window, then on the top right area, there is a box labeled 'Top-Level'. Check this box for every topic that you want available right after the greeting is over. Then also the conditions on these topics can show/hide them depending on what you are trying to do.
Edit: And make the priority of your quest over 50. Also, depending on other quest(s) dialog, when you greet an NPC, your dialog might not show up right away, like if their dialog has choices off the greeting topic, you will only see those. If after that, they have generic greeting/goodbye, your topics will appear.

Thanks! I manage, yesterday, to get it work like that but I wasnt sure about what i did exactly. I had the feeling that Top Level was the key but now, with your reply, i now understand how it works. Thanks!