And one more thing - why is it nor possible to activate plugins like the Datasmith C4D importer? Thanks :)

You can use content downloaded from Unreal Marketplace. There is no technical or legal issue with it. Can you be more specific in which step did you run into trouble?

As for the other question please check this forum topic:

https://my.aximmetry.com/post/284-how-to-import-models-to-unreal-editor-for