You can do this sort of thing by using a CustomControl however when testing just now we found that it will only do a style for small size and not a larger one, which is probably what you'd want for your ListBox.
If you can give some requirements on how you'd like it to work and appear in the ribbon, that would help. We're happy to implement anything to make this work.
This sort of code does embed the ListBox but as mentioned above, it will show up in a small size:
When you reply, please indicate if when adding this to the ribbon (I assume directly under a Group to achieve a large size) you still would like the ImageSource and Label properties to show. The small control size does show the ImageSource and Label if specified. We want to ensure that whatever we implement ends up working for other embedded controls as well. So perhaps if an ImageSource and Label are specified they appear to the left of a large item?
Also we do anticipate a new maintenance release in the near future with some cool new features, including styles for the ScrollBars that appear in ComboBoxes in the Ribbon. You will be able to apply these to your ListBox as well!