You can use existing fields and new fields in creating the index. After you have created an extension of your desired table, you can add an index in the same manner that you would add an index to a new table. You cannot use an extension to create a unique index. That type of development is intrusive and therefore not allowed.
If you set the Created By, Created Data Time, Modified By, or Modified Date Time property to Yes, it will add corresponding tracking information about the user to the table when records are created or updated. Not every property that is listed in the Properties window can be adjusted.