Hi @sbeussink 
Welcome to the Canvas Community. Well....yes and no, and this is a bit complicated. Yes, you can see all scoring rubrics in a class by heading to the Outcomes menu items in a course site (which, by the way, students cannot see) and from the ensuing screen click on Manage Rubrics, which will be on the upper-right. There you will see all rubrics being used in a course that were created in Canvas. (That's important, because since you mention external tool this may very well not apply.)
HOWEVER, on that same screen you will see this important message: Rubrics cannot be modified once they have been used in more than one place. In fact, there will be no EDIT button available unless the rubric was already previously edited. (More on that below.) I encourage you to start with this online lesson from the instructor's guide and continue from there by clicking the NEXT TOPIC link at the bottom, since there is a lot to learn: How do I manage rubrics in a course?
So, as noted above, you cannot change a rubric that has already been used within that Outcomes screen. True. However, you can head into an assignment where a rubric already exists and click the pencil (edit) icon on that rubric. Canvas will give you a warning message, and you can choose to ignore that message and just click OK, where you will be able to edit it. However, the end result of that is that Canvas will create a copy of that rubric by taking the original name and adding a (1) to it. This is covered in more detail here: https://community.canvaslms.com/docs/DOC-12861-4152724129 specifically under the Copy Rubric heading. Believe me, I have seen many instructors with lots of numbers in parentheses next to their rubric who have done this very thing. (You have the added job of explaining it to students after the fact.)
Finally, any changes to that rubric that affect the grade may also result in bad things happening to the external tool involved, especially if the original scoring was done in that external tool and not within Canvas.
Be careful, in other words! But I hope the information above is a bit useful.
