How to get a sparse block matrix?

I have four 3-cross-3 matrices which are sparse. How do i add those matrices to (i,i) (i,j) (j,j) (j,i) of the 3N-cross-3N sparse matrix?

[James Tursa]

We need more details from you. Can you provide a short example showing exactly where the additions are supposed to go in the 3Nx3N matrix? Why can't you just directly add them in?