Hi Aimee!
I'd be happy to share some insight on this!
Drag-and-drop interactions are not currently keyboard accessible in Storyline 360. However, our development team is tracking a feature request that would allow learners that type of functionality. I'll include your voice in the feature report and will update this discussion if it makes it onto our Feature Roadmap.
In the meantime, my colleague Sarah created an accessible download that may help you build the custom interaction you're looking for.
Hope this helps! Please let me know if you have any more questions!