Can you put 2 already existing games into a series? Both games have different moderators. However, if all of them agree, is it possible?

Yes, it's happened before, I think? But it's something you'd want to raise in that thread on the "The Site" sub-forum.

This thread:

http://www.speedrun.com/The_Site/thread/be179

Note that @Lighnat0r says that you need to mod one of the games in order to make the request. I'm not sure if you do or don't, but if you don't, then you can raise your point to one of the moderators.

I am a mod in one in one of the games, but I also know the ohter game´s moderator. And it is true, I should have posted this on "the site" forum, I didn´t realize.