But I don't understand how to modify item and index values.
I don't know what you mean by this. Explain what you are trying to accomplish where you think you must "modify item and index values".

There are two ways to do what you want. First is to derive from your base model (as you have done). The second is to create a proxy model that adds to what the source model provides. I personally prefer to use a proxy rather than derive from the source model. In either case though, you have to pay careful attention to parent-child relationships with model indexes. For selection to work properly, you also have to make sure that mapToSource() and mapFromSource() are correct.