If you have custom catalog data in the older ARI E-Commerce platforms you can easily have it exported and imported by following this guide.
If you are starting from scratch you will have two different options for cataloging your private/custom products and getting them into the E-Commerce Configuration so that you can then list them to your webstore.
Option #1
Create products directly from the user interface
Option #2
Create products by importing a structured csv file into the File Center