It's not so hard to extract some 3d files from it, but the other way around needs some clarification.
Firstly in the tool you need to go over to the settings tab and select your commonworld.bnk folder. this is in your tdu installation -> euro -> bnk -> level -> hawaii. make sure you select that folder and then click save.
Then you want to go find your desired sector by taking this map for reference:
Once you've got one, open up mini bnk manager and go into that same folder, go into Area-# -> Area-#-# and select the desired sector.bnk, click load, and then unpack all.
Now go back to the tdu world editor program, go to import, and within the unpacked folder go to the sector file, 4build -> pc -> euro -> level -> hawai -> area-# -> area-#-# and select the sector-#-#-#-# folder.
If done right, you should see it's recognized the sectors and you want to check 'heightmap' and objects', set the output file as desired, name your file and click import and voila! you should now have a collada file where you set your output to be.
This is where your imagination comes in and you edit the model exactly how you want in your favorite 3d software.
And this is as far as i can go. i haven't found a way to successfully convert the file back as it always gives me an error, but if i find a way, i'll edit this post.