I will try to understand your question. You have 12 integer properties , which would be displayed as 12 columns, You need to add additional row to display the sum of the twelve columns values(Sum of the values in the first row). Whether my understanding correct? Why don't we display the sum as 13th column? It will be easy if we display the pagelist as repeating grid.
We can achieve your requirement using OOTB way. But you need to use embedded sections with repeating dynamic layouts. I believe we can't achieve your requirement with repeating grid layout.