there are different ways to do this. One of which is to define a fully zero matrix with all zeros
Then assign the non zero elements.
newMatrix(1:3,1:3) = oldMatrix;
This is one way to do it.
Another way would be to concatenate the oldMatrix with zeros.
neMatrix_1 = [oldMatrix,zeros(3,2)];
newMatrix_2 = [newMatrix_1;zeros(2,5)];
This should give you the same in newMatrix_2