Is there a way to use the global Cactus reduction functions on fractions of the entire grid?

I am interested in summing grid scalars (say rho) in spherical shells in the grid. Is there a way to perform the "sum" reduction on those shells specified by a global radius variable? Or do I need to write my own MPI function that sums the contributions from all processors that have grid point that fall within the shell I am interested in?

best wishes,

Vassili