Convolution of two arrays with non-uniform grid spacing

I am trying to find the convolution of two arrays: say U and a Gaussian. Both U and the Gaussian are three dimensional. The only problem is that my z dimension is non-uniform in its grid spacing. That is, the dz varies as a function of z. In Matlab's N-dimensional convolution function, I don't see any input for the x, y, and z. This implies that Matlab assumes a uniform grid spacing. What is the best way to compute this three-dimensional convolution where my dx and dy are constant but the dz is changing as a function of z?