Using T-SQL is the best way to go about this, depending on what you
want to see in the DataTable. The following SELECT uses the IsNull
function to convert the null value in ReportsTo to 'Boss'. Note that
you have to use explicit type conversions to convert to a string:

select LastName, IsNull(Cast(ReportsTo AS varchar), 'Boss') from
employees
ORDER BY LastName

The first few rows look like this:
LastName                                            
-------------------- ------------------------------
Buchanan             2
Callahan             2
Davolio              2
Dodsworth            5
Fuller               Boss
King                 5

If you want to see the actual name instead of just the employeeID,
this query uses a self-join to give you the name of the person the
employee id refers to while using 'Boss' instead of null:

SELECT Employees.FirstName + ' ' + Employees.LastName AS EmployeeName,
 IsNull(Cast(Employees2.FirstName + ' ' + Employees2.LastName AS
varchar), 'Boss') AS ReportsToName
FROM Employees LEFT JOIN Employees AS Employees2
 ON Employees.ReportsTo = Employees2.EmployeeID
ORDER BY Employees.LastName

The first few rows of the result set look like:
EmployeeName                    ReportsToName                  
------------------------------- ------------------------------
Steven Buchanan                 Andrew Fuller
Laura Callahan                  Andrew Fuller
Nancy Davolio                   Andrew Fuller
Anne Dodsworth                  Steven Buchanan
Andrew Fuller                   Boss

--Mary