In Moodle, there is a gradebook for each course. If you have a course in another system, and no corresponding place in Moodle, then just create an empty Moodle course, so that you have a corresponding gradebook.
A Moodle gradebook is basically a spreadsheet with one row for each student, and one column for each graded activity. Those columns may relate to a Moodle activity, like an assignment. You can also add extra custom columns.
You can import grades into the grade book in a range of formats, the simplest of which is a CSV file. That is probably the way to get grades from your other system.