list manipulation - Creating overlapped block matrices

I'm doing an FEM assignment using Mathematica.

(EK1 = {{a11, a12}, {a21, a22}}) // MatrixForm
(EK2 = {{b22, b23}, {b32, b33}}) // MatrixForm

I don't know how the best way to create new matrix like this:

(K = {{a11, a12, 0}, {a21, a22 + b22, b23}, {0, b32, b33}}) // MatrixForm

MATLAB does it quite easily but how about Mathematica?