Variable Expressions In Derived Columns - Evaluate ColumnNames??

Trying to setup a derived column to use an expression stored in a package variable. But it seems that variables are always evaluated as text...I need it to evaluate as a Columname sometimes.

Example:
On an ETL of Products I want a new derived column that uses two other columns.

I can hardcode an expression of ProductID + " - " + ProductName and that results in dynamic output. But now I want to use that as an expression stored in a variable so I can change it when needed.

So I make that expression = @[User::Variable]
and stuff into @variable ( a string param) : ProductID + " - " + ProductName
My output is the literal "ProductID + " - " + ProductName", and not the actual ID's and Names

I've tried with/without brackets, quotes and braces but no change.


Any way I can get the pieces in that variable expression to evaluate as column names?