NetAppend
NetAppend[net,layer] appends a layer or a net onto a NetChain, a layer or a NetGraph with one output port.
NetAppend[net,"name"->layer] appends a layer with a given name.
NetAppend[net,{layer1,layer2,…}] appends several layers or nets.
Please visit the official Wolfram Language Reference for more details and examples on core symbols.