cornice

I'll help you figure this out. Lightroom desktop itself cannot add borders when exporting images.

There are other ways you can do this.

1. You can send images to Edit-in Photoshop to add borders. Check this to get started: https://helpx.adobe.com/lightroom-cc/how-to/lightroom-photoshop-edit-photos.html
2. Lightroom Mobile (iOS - iPhone) can add borders at export. Here is the feature request: https://community.adobe.com/t5/lightroom-ecosystem-cloud-based-ideas/p-ability-to-add-a-border/idi-p/12232868
   
   Let me know if this helps. Thanks!
   Sameer K