Enrolment

Hi there, I'm a developer and new to Moodle. I've search for days now to find a solution for the same thing you are looking for. If you managed to sort out your problem, could you please help me on how to add a user directly into the Moodle db and assigning them to a already created course. The scenario is like this. My client has a separate website where he advertises his courses. He also has a Moodle account/website with his current courses and students. Now when people goes to his separate website, they can select a course and register. I then have to take this user through a payment process and then manully insert them into my client's Moodle site and manually enroll them to this spesific course. Could you please help me with this. I got the part working where I need to insert the user into the Moodle db, but the enrollemnt part is unfimiliar to me. Thank you!!