Yes, absolutely! The gamemode property that you set in the world settings is just the default gamemode, for testing. When you've finished the map, you'll just have to let me know which game modes your map supports and when I add it to the game, I just set it so it can be used with any of those game modes.
As for actually supporting other game modes in the map, that's just a case of having team specfic spawns for team deathmatch, and if you want to support Capture the Sphere, you need to place a sphere in the level e.t.c, e.t.c. I'm afraid I havn't created the support material yet on supporting capture the sphere or king of the hill, but I'm planning to make a video on this today if I have time.

What do you mean? I would guess that a new gamemode for what you have in mind would simply need to be added; You could try making it yourself, but I assume the game wouldn't recognise it properly since only the map itself has it.
EDIT: lol I'm too slow XD; also, I foolishly missed the obvious interpretation of the question

A complementary queston: If you wanted a map to support KotH & DeathMatch & Free-for-all for example, could that be done? (Since the KotH zone would need to be removed in DeathMatch and Free-for-all)

Hi Rakurai, yep, you can do that. KOTH zones are automatically removed when you start a match of another game mode type, so you can just place the KOTH zone in the level. Chamber supports koth and deathmatch for that matter. CTS spheres and goals arn't removed though, just because I thought CTS maps probably won't be suitible for any other game modes anyway, since they'll probably be quite long and linear

I've just added a section to the guide on supporting different game modes. When I thought about it, it made more sense in text than as a video.

Thank you.