at the moment unfortunately you can’t, because the design mode of QtCreator is only usable for the built-in QML elements like Image, Rectangle, Column, etc.
However, we are working very hard on a new feature of V-Play, which will allow you to drag your objects into the game, and to change the properties of your objects while the game is running! You can then save the levels to your hard drive and load and continue modifying them at any time. The cool thing is, you can also add the level editor functionality to your released game, which allows your players to create content by themselves and so build a community around yor game!
We expect to release this feature in January, so I hope you are fine with adding your entities manually in the code edtor so far.
Can you think of any particulare features you would like to see in such a level editor? We have a bunch of ideas internally, but it would be great to hear what you would like to have.