Honestly spent the last 6 hours trying to find answers online and/or figure it out myself to no avail. I found several posts that mentioned it as if it was common knowledge. One that outright said it was possible And one thread that detailed how to get a demo in UT editor but the process did not work for me. I only spent about $65 so not the end of the world to me, but I was really hoping to use this as a stepping stone to help me learn this iteration of unreal tournament editor.
In the Epic Games Launcher under Library and Depot there should be the list of things you bought from marketplace. You can click “add to project” there. Is it that what you meant?
No. That adds to the unreal engine only. Supposedly you can migrate the assets to unreal tournament editor. And you can kinda, but it doesn’t work. I just can’t find any info on the subject that actually works.
Have you Tried Creating an Empty Project With all those assets and Then Copying The Assets (The .uasset files) from UE4 project into your UT project . i have never used UT editor . But i think this might work . If textures and Materials aren’t loading give it some time . if you still have the same problem restart the editor 
UT doesn’t use projects as such…
Basically UT lacks the entire ‘project’ system. While you can make more than just levels, it isn’t the same structure as the unreal engine.
What i meant is create an Empty project in UE4 outside of UT , Then drag the UAssets to your UT thing …
This is a bit old topic, bu can i do it legally? I bought bunch of asset packs on unreal engine marketplace, can i use them for creating my custom level for UT? Do i need contact authors and ask for permission? Technically they are sold to make my own games, and ut level is not my game. I also worry that those assets can be easily extracted from ut level.