You do not need to purchase an extension.
If you do not take or pass your Final Exam (three attempts) within 365 days from your purchase date your examination attempts will expire. You may schedule another examination three attempts by purchasing a retest (at an additional cost) at any time after your testing period has expired or you have used your three attempts.
You can purchase examination retests by contacting the NASM BOC at 800.460.6276, Option 3.