Dear student,
I guess you are asking about jee mains. Then yes you can register for other sessions separately when the application window will open for them. The application window for March session will open in the last week if February, for April, in the last week of March and for may in the first week of may tentatively. You are advised to be updated about the dates so that you do not miss the dates. You can either apply for all the Sessions at a single time or apply separately.