Casting to Custom Classes

[SMFX]

Okay, this should be a simple thing, but I can't seemto find the right
documentation on it. I created a custom class that inherits from a well
known class. For example:
Class MyString Inherits String
Public Sub New()
MyBase.New()
'custom code here
End Sub
'Custom properties here
'Custom subs and functions here
End Class

Now, what I would like to be able to do is cast from the well known public
class to my new custom subclass. Do something like this:
Dim wellKnown As String
Dim myCustom As MyString

wellKnown = "Some string"
myCustom = wellKnown
'or if the above won't work
myCustom = CType(wellKnown, MyString)

Then if I'm really daring, be able to go back the other way:
wellKnown = myCustom
'or if need be
wellKnown = CType(myCustom, String)

Seems like it should be a simple action for inheritance, but I guess I'm
missing the obvious if so. I know that it means some values may not be
populated, but I'd like to gracefully handle it.

Any help would be greatly appreciated!

TIA,
SMFX