Using enum as integer constant in C#

My question is pretty simple, but I didn't find a way to implement my code the way I want it to be. So I started wondering if the code I want to implement is not good. And if it is, what's the best way to do it.

Here it goes:

class InputManager  
{  
    SortedDictionary<ushort,Keys> inputList = new SortedDictionary<ushort,Keys>();  

    public void Add(ushort id, Keys key) {...}  
    public bool IsPressed(ushort id) {...}  
}  

class Main  
{  
    private enum RegisteredInput : ushort  
    {  
        Up,  
        Down,  
        Confirm  
    }  

    public Main()  
    {  
            InputManager manager = new InputManager();

            manager.Add(RegisteredInput.Up, Keys.Q);
            manager.Add(RegisteredInput.Down, Keys.A);
            manager.Add(RegisteredInput.Confirm, Keys.Enter);
    }

    void update()
    {
    if(manager.IsPressed(RegisteredInput.Up)) action();
    }
}

This code won't compile, giving errors of this kind:

The best overloaded method match for 'InputManager.Add(ushort, Keys)' has some invalid arguments
Argument '1': cannot convert from 'RegisteredInput' to 'ushort'

If I use a cast like in manager.Add((ushort)RegisteredInput.Up, Keys.Q); it will work. But because the cast must be explicit, I was wondering if it is not recomended code in C# like it is in C++ and if there is a better way of doing it (like using const ushort for every value, which I kinda don't like much).

The best answer I got so far was from this thread, but it sounds so much like a hack, I got worried.

Thanks!