In fact, i don't think there is a convexity formula in Excel, although duration has.
So for your problem, maybe we can work it out as followed:
1. Numerical method. Calculate the durations of yield and shocked yield, then take the difference to get convexity.
2. Definition. Calculate all convexity contributions of all cashflow and take a sum. Just like convexity formula.
3. write a VBA code!

1. is fast, 2 is complicated but intuitive, 3 is "one kill for all"

原来可是这样

[sad]