What do you mean it gives you the option to add more items? I'm assuming you are talking about the preview feature? Each item is separate from the next. When you compile your model in-game, it automatically creates a .mdl file from your .smd. The feature you are looking at simply allows you to preview multiple items on a character at once.

The game compiles your .mdl files to the C:\Program Files\Steam\SteamApps\common\dota 2 beta\dota\models\items\<heroname>\<numbers> folder.

Thank you, thats what i needed to know