Good evening!
In my meager experience, and as verified by a helpful forum member here (thanks again, alec!), there is no way to sort sender identities. If it's not too much trouble, it's a feature I'd love to see. I'm fine with simple alphabetical options, but drag-and-drop like the folder options interface might work better for some.
While I've got your attention, thanks for all the work on Roundcube! It's a few cuts above the other options I had available, and my computer-adverse relatives actually like it! Keep up the great work!
Caslomyr