NGUI has an inventory example. Check out the "Example X - Character" demo scene.
It's simple enough to add your own items. All you have to do is add them to your atlas and then set the icon to whatever sprite name you want.
When you are in the scene, take a look at the ItemDatabase object, it's easy to click "New Item" and define it how you like.
I don't understand the need to generate an atlas at runtime. If all of your images are going to present when to do the build, it's easy enough to add them to an atlas and then assign your sprites to the newly minted items in the inventory database. Then you won't have the extra overhead trying to dynamically generate content.
Hope that helps.
