How can I attach a Gameobject from the scene to a prefab on the Assets folder?

The thing is that I have been doing a multiplayer game, but when I used the Network manager of unity, I needed to put a player prefab. But my problem is when the player became a prefab, the gameobjects that I've attached to them through the inspector gets removed and I cannot reattach the gameobjects from the hierarchy tab (the scene). How can I attach the gameobjects of the scene in a prefab?