Mikand79, yes. You can start distributing game mod tools including customized versions of the Unreal Editor and even source through forks of UE4 on GitHub, and is fully supported right now.
We are also working on a more user-friendly framework for distributing these things through the Marketplace. Check out the Unreal Tournament tab to see the current state of things.
John, we recommend that each game project customize the editor as appropriate for its modding needs, because the tradeoff on features versus ease of use will depend on the project. Check out the UT Editor, which has some customizations compared to the base Unreal Editor to help guide users down the right path, including removing the “New Project” functionality (which breaks the UT workflow) and adding a new “Share” button.
Unreal Tournament provides an example of “modding best practices” based on the current state of the art, though we’ll be doing a lot more in area throughout 2015.