How can a client connect more players to a network game?

Currently my game can controlled on one keyboard by up to 8 players, without any networking involved. Now I want to extend the game with networking. So let's say 3 players are on the server-client and other 5 players connect from the client.

As far as I know it is possible with UNET that more players connect by the same client. In the network manager I selected the player prefab (which has its network identity) and turned off "Auto Create Player". So I need to create the appropriate number of players myself. Although I can't find the right method for that.

The closest I've found is that I instantiate the object and connect it with the NetworkServer.AddPlayerForConnection() function. Although it needs a NetworkConnection type parameter and I can not find in the documentation where can I get its value.

Am I close or does my problem need a totally different approach? How can I connect more players with one client?