Yes, but please consider that the course will then be more expensive. Basically: the longer you register for, the lower the price. For example if you register for 24 weeks at the start, you pay 3,310 €. If you register at first for 4 weeks, and then extend the course month by month, you pay 3,680 €.
However, please keep in mind that you cannot cancel or shorten the course after it has started. Thus, if you are unsure about how long you can actually stay, it is better to book a shorter course period first and extend the course later. Otherwise you will lose the course payment because we cannot refund you if you discontinue the course.
You might be able to take out insurance against course discontinuation in your home country before you travel.