You can easily do this using Transformation variables.
In the dialog box for the Distribution, simply right click and create a virtual transformational variable; If you specify the formula as in the example, you can use that new virtual variable for the distribution.
To do this is JSL, here is an example
Distribution(
Transform Column(
"sex expanded",
Nominal,
Set Property( "Value Order", {Numerical Order( 0 )} ),
Formula( If( :sex == "", "Missing", :sex ) )
),
Nominal Distribution( Column( :sex expanded ) )
);
Jim