If my understanding is correct, you want G2 to add itself to D2, every time D2 is changed.
This will require a VBA solution, a cell can onlt contain a value OR a formula, it cant contain both
I will move this to the VBA forum for you

As a side note, I see that you have each of those 3 "cells" merged over a 3-column x 6-row range. This may just be for the purposes of your sample file, but if not, I would strongly advise against the use of merged cells, they cause all sorts of problems for formulas