Yes, you can purchase up to 25000 miles at USD $40 per 1000 miles:
http://www.emirates.com/english/help...purchase-miles

However your ticket class (Saver / Flex / Flex Plus) will have a bearing on the miles-cost to upgrade. Assuming that you only have 8700 skywards miles to your name, you will require a total of 30,000 miles to upgrade based on Flex Plus ticket flying LHR-DXB. If you've got a lower-class ticket (Saver or Flex) then a miles upgrade will be out of the question as you wont be able to purchase enough miles to meet the total upgrade miles required.

Also take into consideration (if you're looking at purchasing miles) is that the ability to upgrade with miles is never guaranteed - again assuming you are flying out of LHR which is one of the busier legs.

Rather than spending the money on miles, you could see what the cost is to re-grade your ticket at check-in.