You can't add a transition to your videos until they're on the timeline. Transitions can't be added in the Project Assets panel.
I would agree that you cannot use Timeline Menu/Apply Default Transition unless you have the clips to which to apply the default transition on the Timeline.
But, you can add default transition from the Project Assets panel.
1. Import your files into the Expert workspace's Projects Assets
2. In Project Assets, select all the files to which you want to apply the default transition all all one time
3. Right click anywhere in the selection and select Create Slideshow
The Create Slideshow will bring up the Create Slideshow dialog
Make sure to have a check mark next to Apply Default Transition. In this case, I had beforehand set the default transition as Random Invert.
Click OK when finished in dialog. Result will be the selected Project Assets files on the Timeline with default transition placed.
Please review and consider, and then let us know if the above will fit into your workflow goals.
How do you set a transition to "Random Invert"? I have Premier Elements and Invert isn't a choice, let alone Random.
There is no random invert that I know of.