Hi @jokilroy ...
While you cannot have the Turnitin interface and a Canvas rubric displaying on the screen at the same time in a given assignment, you can still build the Canvas rubric and assign it to the assignment as you normally would before enabling the Turnitin interface. Once that is all saved, then you can go back into the editing mode of the assignment and switch it to use the Turnitin LTI. The rubric will still be there...just hidden. Students could still go to "Grades", click on the assignment name, and then click on the "Show Rubric" link at the top right corner of the "Submission Details" screen.
As you've mentioned, you can input the rubric into the Turnitin interface. The nice thing about that is that when it comes time to grade an assignment, you can do it from the Turnitin interface, and the score gets passed back to the Canvas gradebook.
Also, you might want to take a look at this:
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