A student did his paper in googledocs but didn't give me access to it; he then sent it as a .pdf. Can I submit the paper into the assignment slot for that student so that I can use my rubric to grade it?
No, but there is a feature request to do this that is currently on Product Radar. You may want to head there and lend your support: Submit an assignment on behalf of a student
Note that you do not have to have a submission to use the Canvas rubrics in SpeedGrader. I still fill out a rubric (even though it gets 0 points) when students don't turn in an assignment. While rubrics don't require a submission, if you wanted to mark up and annotate the document using DocViewer, then you would need a submission.
Retrieving data ...