Can I create a competition on editor

Like world cup with more than just a league with 2nd round final etc?

Creating competition is, among others, what editor is for. But I'm not sure about world cup, and I'm even less sure what you mean with 'world cup with more than just a league and 2nd round final'.

what i mean is like create a competition which goes through various stages e.g after first group stage goes to a knockout round and so on till final

Yes you can

how though if i add nation rules to do a international league how do i set the rounds up ?

First of all, I think you'd better ask this in the editor forum, and read the many posts there, they will help you.

You can create international tournament just like you add domestic tournament. So far, your only choice with nation rules is to add the international tournament to a nation, you can not add it to "world" or "continent" in nation rules editor. It doesn't always work because you have to make sure it doesn't clash with international tournaments already in the game.

For a tournament with group stage then knockout round, you add it as a 'cup' which use a group stage, then you should determine how many team would go to the knockout round, and finally set the knockout rounds themselves.

Creating international competition is not really the same matter with creating club competition, you have to be careful because nations don't behave exactly the same as club, they are linked to several other international tournaments and dates. For a more sophisticated international tournament with more options, you have to use advanced panel and not nation rules. Again, you should take a look at the Editors Hideaway forum to learn more about this.

Finally, if this is your first time using the editor, I recommend you to try creating a simple domestic league and cup system first just to understand how the editor work.