I'm using WooCommerce for my shop but on the product page i need to add a dropdown box or something to allow a user to add an additional item to the product if needed. Is this easy to do?

WooCommerce does let you create grouped products, where you can add one product to another (e.g. if building a computer, you can add a component to the order).

 

You could also go down the route of setting it as a variable product, and letting them add the additional item as an add-on.