Is there a way to make Premiere reuse nested previews?
I don't think there is a way to accomplish this, since you can apply effects on nested sequences as well as to single clips, which means the nested sequence might look different in the main sequence than it does in the nested one. My humble guess is that that is also the reason why Premiere does not use the existing preview files.

A workaround would be to export the nested sequence and re-import the rendered file in your project, replacing the nested sequence. This of course means you won't be able to make changes to that part of your video anymore, as well as a slight loss in quality (unless you export the sequence using a visually lossless codec such as ProRes or DNxHD) ...