Technically technically, Charmed isn’t a status effect, it’s an instant mechanic. There is no persistence to it. You could argue that a troop that is Charmed is inflicted with a 0-length status effect that causes it to attack and then be immediately cleansed, but that isn’t how it is implemented.
I’m with @Ashasekayi on this one. Impervious is already extremely strong. If we find a need for Charm immunity, then perhaps a new trait is in order (although frankly, once Wisps are finally dealt with, such a trait becomes subpar).