I am looking to install cubecart into my website and would like to know if there is a feature for upselling items available? eg. A dropdown list that asks: "Would you like to add....." or similar?

If you are refering to a wish list, then No, but you can find this 3rd party mod in .org and plenty of more mods in there.
Maybe find something you might need, have a look

There is an add to cart (by default) that will allow you to add a product to the basket.

Hope i got the question right and gave you a proper answer

There is a "related products" mod on .org This currently won't do what you want, but will display a list of products that are related the one being viewed.
This can automatically "guess" related products, or you can manually add them.

With a mod to that mod (The author should be able to help), you should be able to get it going..

I use the related products mod on my gear4mobiles.co.uk store in automatic mode.

Hope this helps,
Jason