This is Dallas here from the Shopify Social Care Team.
That is a great idea! What you would need for that would be a Bundle app. One of my personal favorites is called Bold Bundles. This won't give your customers the option to add more items while they are looking in the cart, but this option will be presented to them on the product page.
Although there aren't bundles for the cart you are able to add cart pop ups! This will allow you to pass along any messages to your customers that you want/need to.
I hope these help! If you are looking for something different or would like some more help with any other questions please let me know.
Recommend Personizely (I work here). You can set up the target with the condition "cart contains - 1 item in cart" and a popup will appear only if the cart contains one item. Also, you can set up the target by "cart value" and a popup will appear only if a visitor's cart total value is more/less, for example than $100.
If you have any questions don't hesitate to contact me via email: email@example.com