Hi,

how can I edit the Players skills? Like Speed, Strengh etc.

In sandbox or multiplayer mode click on a player to get his player card to display and click edit player. This is not available in career mode.

Ok, thanks.

But it is possible to adjust the Rosters so they are modded for every new started savegame?

Yes, you can export rosters either to edit them and re-import them for an existing game, or to use the export as a starter for a new game.

When I Export them, I have a .csv data. To Import them I need a .json data. How can I do that?

There is a converter in the mod forum with other examples from folks building mods. I would check there and see if what you're looking to do already exists first.