If you want to insert some modules dynamically, try nn.ModuleList instead of nn.Sequential. Some operators such as nn.ModuleList.append, nn.ModuleList.extend, nn.ModuleList.insert may helpful.
In my understanding, do you want to dynamically choose whether to use skip connection(ADD)? I have an idea
self.door = 1  # when you define your model
S11 = C12 + C * self.door # in forward function

net.door = 0 # if you want to drop the skip connection
net.door = 1 # if you want to keep the skip connection