To add the matrices together, just arrange them by indices.
% Dummy values for k1 and k2
k1 = ones(4,4);
k2 = ones(4,4) * 2;
% Calculate stiffness matrix
K = zeros(6,6);
K(1:4, 1:4) = k1;
K(3:6, 3:6) = K(3:6,3:6) + k2
You can make this more robust by using variables instead of hard coding the indices, but this minimal example shows the concept. Good luck with your finite elements!
