ZoomCommand = new RoutedCommand( "ZoomCommand", typeof( MyCustomControl) );
this.CommandBindings.Add( new CommandBinding( ZoomCommand, zoomCommandHandler,
    ( s, e ) => e.CanExecute = this.IsZoomEnabled ) );

<KeyBinding Key="Add" CommandTarget="{Binding ElementName=myCustomControl}"
            Command="{Binding ElementName=myCustomControl, Path=ZoomCommand}" />