My course is year-long and I need to extend it (to the following semester). Do I use CDMS Course Combination tool?
No, this has nothing to do with CDMS.
In your Canvas course: Proceed to your course settings > extend the course end date accordingly > Save.
Reference guide: Change course end date
You can only do this while the course is still ‘active’ (have not reached course end date). Once you have went past the course end date, please request to CTLT Service Desk for assistance: https://nus.atlassian.net/servicedesk/customer/portal/4/group/5/create/17