Hi @DanHahn ...

It is possible to open up a course just for one student...if that is what you are wanting to do.  When we get requests from our faculty who have students who need more time, we do the following as Canvas administrators:

1. In the course, click on "Settings", and then click on the "Sections" tab.
2. Create a brand new section...we call it "Extension".
3. Click on the name of the new section you just created.
4. Click on the "Edit Section" button.
5. Put in Start/End dates for the period of time you would like for your student to have the extension.
6. Make sure to click the checkbox for "Students can only participate in the course between these dates".
7. Click on the "Update Section" button.
8. On course "People" page, click on the three-dot kebab icon to the right of a student's name, and select "Edit Sections".
9. Click the "browse" link to find the section name (ex: "Extension") that you created in Step #2, and then click the "Update" button.
10. Repeat Steps #8 and #9 for the instructor of the course, too.

If you are a Canvas admin, you could then "act as" (masquerade) as the student to make sure that this person has access to the course...and that the instructor could change the grade.

Hope this helps a bit.  If I've missed the boat on what you are asking, please come back to this thread to provide additional information so we can continue to help you.  Thanks!