You only need to pass the values for each column of the row to that method, not a map containing name/value pairs.
In your case it'll look something like this:
IDataTable table = Core.retrieveOQLDataTable(this.getContext(), query);
for (IDataRow row : table.getRows()) {
String employeeID = row.getValue(this.getContext(), Employee.MemberNames.EmployeeID.toString());
String firstName = row.getValue(this.getContext(), Employee.MemberNames.FirstName.toString());
result.addRow(employeeID, firstName);
}