How to add additive noise to an image?
You don't have to go to the frequency domain to add noise. I'm not completely sure what you mean by adding it, because those number in N are quite big. Are those variances or actually sample values?

Adding them together can simply be done by well... adding them together (value by value). Since they both have the same size, this is possible.

Normally one would give a noise distribution with its paramters. Those random numbers can then be generated and added to each element of the matrix.