Inherit enum members

[Andrey via Digitalmars-d-learn]

Hello,
I have got 2 enums. How to inherit one enum from another?

enum Key : string
{
    K1 = "qwerty",
    K2 = "asdfgh"
}

enum ExtendedKey : Key
{
    E1 = "q1",
    E2 = "w2",
    E3 = "e3"
}

Result:

onlineapp.d(27): Error: cannot implicitly convert expression "q1" of type string to Key onlineapp.d(28): Error: cannot implicitly convert expression "w2" of type string to Key onlineapp.d(29): Error: cannot implicitly convert expression "e3" of type string to Key

How to understand this?