How are you calling these methods?
The "conversation" variable is an object specific to the current conversation. You can store any data on it and it will be persisted with the conversation and accessible for the duration of the conversation.
Make sure you are no calling init() after add_item() as that would reset the itemList attribute to an empty array.
You can also store data on the user object using the "speaker" variable, or globally on the bot symbol #self, or globally on any other symbol your choose using #symbolName.
Here is an example of a script that uses the conversation variable to store data.