Hi @CCOP
Do you have (or can you generate) spreadsheets of the users, courses, and enrollments?
It's quite possible to use the "SIS Upload" tool to manage users, courses, and enrollments even if you don't have an SIS. The critical thing to understand is that the "SIS_ID" is just the unique identifier for users and courses that will not change. In most institutions, it would be a student (or faculty/staff) ID number for users, and a course/section/term identifier for courses.
The SIS Import makes use of the SIS_ID field to match the users in the users and enrollment feed files, and to match the courses in the courses and enrollments feed files, so there must be an SIS ID in order for the process to recognize the person or course. You can go back and manually add an SIS_ID to users and courses which don't have one, and that will allow them to be managed by the SIS Import process.
There is some introductory information on formatting feed files in CSV format to be imported via the SIS Import tool at https://community.canvaslms.com/t5/Admin-Guide/How-do-I-format-CSV-text-files-for-uploading-SIS-data...