Is it possible to create one course in moodle and add many other courses to it?

Question

I am asked by a client to create one course on moodle and add many other courses to it for one instructor to handle it. So that the teacher can grant students access to it at anytime he wants

Answer 1

Totara (a fork of Moodle) and Moodle Workplace both have programs / learning paths for multiple courses

But they are not open source

Totara Programs

https://totara.help/docs/programs-and-certifications

Moodle Workplace Programs

https://docs.moodle.org/402/en/Programs

Answer 2

Your request is a bit vague, but try the Subcourse activity plugin.

"Subcourse behaves as a graded activity. The grade for each student is automatically fetched from a final grade in another course. This allows course designers to organize the learning content into separate units."

https://moodle.org/plugins/mod_subcourse