This is a bit tricky. First of all, you should declare the property like any other property. Then you must instruct the SQL producer to declare a formula on that column. You can do that with a custom 'compute' attribute in the SQL producer namespace. You can set it with the Visual Studio modeler like this:

In this example I've created an int property that is just another column value multiplied by 2.

Optionally, you can declare the property to be 'read on save' because most of the time, you want to read the computed value after a save, not only on load operations:

Once this is all done, this sample console app should display 30:

class Program
{
    static void Main(string[] args)
    {
        var c = new Customer();
        c.Name = "killroy";
        c.Age = 15;
        c.Save();
        Console.WriteLine(c.Age2); // will display 30
    }
}