Hi Carolin,
Input argument of fitnet function takes array as an input. This array contains the number of neuron to be set up in the model. This function is not yet customized to add different custom layer. There is a way you can add dropout layer, but things will become more tedious to handle. Customization of fitnet function can be done by accessing the output object, redefine the Layers property in the object and use it. Still I would suggest to just use trainNetwork, It gives you more flexibility to choose layer and even customize it.
I hope it helps.
