No, you cannot purchase extra credits. You will have to figure out which of your meals will be the least expensive and pay for those OOP. We have found breakfasts are often the least expensive, although some TS restaurants are also fairly inexpensive. Also, there have been times when someone in our group wanted an appetizer (OOP) rather than an entree so that's another way to save a credit.
Finally depending on what 2 credit meal you are thinking of, sometimes when you divide the cost by the 2 credits it turns out not to be a good use of 2 credits. We found this to be the case at CRT.