Magento – How to Add Product to Cart Programmatically via Observer

Here is my questions: What is the right event to hook in for adding a product to quote programmatically (entry sales_flat_quote_item table). Also i have to catch the items/products added to cart from an user/customer, because their data informations will determine the product which will be added programmatically. So the scenario would be:

1. user/customer adds a product to cart
2. find the right event for hook in
3. fetch informations about the products added to cart
4. add additional product to cart based on a product id and modified its data

In my opinion its better to work with the quote, before the products are written to the database.

I figure out how to add it through Mage_Sales_Model_Quote::_addCatalogProduct(). But i would like to do it through an event observer instead of overwriting core classes.