You could try the following expression
min(date) over(partition by customerID)
This may or may not work in a calculated column of the DSV. If it does not work as a calculated column, you could use this expression in a named query instead. To do this, right click on the table in the DSV, and select Replace table/with named query. This shows up a SQL select statement. In this, you can add the above expression, and it should work in any case.