People that are affiliated with the University, but not enrolled, can be added to Canvas courses but not via SIS.  The instructions are here:  https://kb.wisc.edu/luwmad/page.php?id=66202

Employees who wish to audit a class need to be special students, they can request that status here:  https://acsss.wisc.edu/special-student-types/  They will not be able to enroll until the class starts.