I need to create a field to define if a field is selected or not.
And I had thought about using an "InternalCalc" type field, but the dataset would not seem to allow me to insert it and even if at a later time I assign the type in the opening phase of the Dataset it gives me an error.
If the field is read only you can use a calculated field.
If the field is read/write, then it should be saved in the object itself. You should either create a property in the object to hold that information, or create a class specifically to use in the dataset, and populate it manually.