Can you add a new field to an existing group in a pivot table?

I have a pivot table that I group by month & year that I refresh at the beginning of each month, well sometimes I forget to type some information for Dec 2012 & when I refresh the pivot table its not part of the Dec 2012 group. The only way I know to add it in is to ungroup the whole month of Dec 2012 and then regroup... it takes a while... any way around that???