Add Guest Order to a Customer Account in Magento

To get the customer_id simply go to Customers > Manage Customers in the Magento admin.

Log in to the MySQL and execute the following queries:

UPDATE sales_flat_order 
 SET customer_id = (YOUR-CUSTOMER-ID), customer_is_guest=0 
 WHERE entity_id = YOUR-ORDER-ID AND customer_id IS NULL
 

UPDATE sales_flat_order_grid 
 SET customer_id = YOUR-CUSTOMER-ID
 WHERE entity_id = YOUR-ORDER-ID AND customer_id IS NULL