As far as things go right now, it is impossible (or documented badly enough that it feels so) to import a list of topics from a regular text file. Being able to import a list of topics from another InDesign file is useful, although quite limited: the true function that is needed is the one useful at the time of CREATION of the index, not transfer from one book to another. Importing lists of topics MUST be made much clearer, easier, and simply just WORKING. This is what is expected from a piece of software like InDesign.
As far as things go right now, it is impossible (or documented badly enough that it feels so) to import a list of topics from a regular text file. Being able to import a list of topics from another InDesign file is useful, although quite limited: the true function that is needed is the one useful at the time of CREATION of the index, not transfer from one book to another. Importing lists of topics MUST be made much clearer, easier, and simply just WORKING. This is what is expected from a piece of software like InDesign.