Supplemment plans are referred to as "Medigap" plans. Medigap plans are sold by insurance companies to cover services for which Original Medicare does not cover, or only partially covers. This may include co-insurance, co-payments, deductibles and the "gaps" between what Medicare pays and the total expenses.

You get a 6 month “Medigap Open Enrollment” period, which starts the first month you have Medicare Part B and you’re 65 or older.  During this time, you can enroll in any Medigap policy and the insurance company can’t deny you coverage due to pre-existing health problems. After this period, you may not be able to buy a Medigap policy, or it may cost more. 

Your Medigap Open Enrollment Period is a one-time enrollment.  It doesn’t repeat every year, like the Medicare Open Enrollment Period.