What do you mean by 'variable control'? Do you mean a column name is contained in a variable called control?
Here I am just guessing at what you are looking for, maybe something here helps get you started?
Names default to here(1);
//Open some sample data
dt = Open( "$Sample_data/big class.jmp" );
//Delete some values
for(i=1, i<= 20, i++,
Column( dt, random integer(4,5))[random integer(40)] = .
);
//Get column names in a list/variable
cols = dt << Get Column Names( "String" );
//Create a new column referencing the last two columns
c = n items(cols);
Eval( Eval Expr(
New Column( "New Value", Numeric, "Continuous", Format( "Best", 12 ),
Formula( If(
!Is Missing( As Column( Expr( cols[c] )) ),
As Column( Expr( cols[c] ) ),
!Is Missing( As Column( Expr( cols[c-1] ) ) ),
As Column( Expr( cols[c-1] ) ),
.
) )
)
) );