It all depends on what you are trying to do exactly. You can add WC players to your current roster, even activate the new real faces that come with WC mode. Even use the new stadiums.
But adding the entire WC mode to your db and making it playable? I doubt it. Which can't be what you want anyway, because I assume you can already play WC mode friendlies online right?
Seems you want a squads save of WC mode, and you assume it will work online. It's a big assumption I'd say. Anyway, explain what you want to do with it, and I will write a few ideas down to try.
No, I don't want to add WC players to non WC mode. I want to do the exact opposite [which is the main issue here].
I want to add some non wc players on wc mode itself. I can do it with editing the wc db, right? I mean I should be able to do it.
But I wanted to add the non wc players to wc mode without editing the db and only way to do that is if I can edit a squad file of wc mode. But, so far FiFA 18 doesn't generate any squad file for wc mode. So if I can't even create a squad file from the wc mode itself, then its not even possible to proceed any further.
The key point here is "From Non WC to WC", not "wc to non wc". I want to add players on the world cup mode.
I don't want to play online with my edited squads. The reason editing squads [without editing db] works on non wc online modes is because:
When I play seasons, FUT or anything, the game downloads updated squads specifically for online. So it doesn't clash with my offline edited squads and it doesn't clash with db since I haven't touched the main db files.
This gives lots of advantages. I don't have to constantly switch between non modded fifa and modded fifa to play online and you aren't at the risk of getting banned for even trying online since nothing's modified on the main files.
That's why I find editing "squads" file more important and reasonable than editing db.