are graphs/networks additive

Question

I was wondering if networks/graphs are the sum of their parts. Let's say you have a 15-node network. The spectral density of that network has X kurtosis and Y skewness. You also have a 20-node network, with A kurtosis and B skewness. If you append one network to the other at one node (so there's just one edge between the two networks), will the spectral density of the new 35-node network have 0.75*X+A kurtosis and 0.75*Y+B skewness? (0.75*X+A)/2 kurtosis and (0.75*Y+B)/2 skewness? Are there any predictions we can make about the form of the spectral density for the combined network?