I found what is for me the easiest solution to export an animation to Premiere Pro
First, I create a new project in Animator, I click on Settings and i create a colored background (blue or green, depending on the colors already in the picture, with further chroma keying in mind)
Then i work on the project, up to final phase (video export)
Then i import the project into Premiere Pro, I add a chroma keying effect to the clip, and I erase the background color.
It's easy, it works fine