How To Quickly Add Borders When Exporting Photos From Lightroom (Without Using the Print Module)
It’s a mystery as to why Lightroom doesn’t offer a quick and easy way to add borders to images when exporting files. However, there is a simple solution. While Lightroom’s export dialogue does grant you a large amount of control, the option to add a simple border to your files is notably missing. Anyone wanting to achieve this classic look is typically forced into one of two routes: The first option is Photoshop, which offers automation through Actions and Droplets, but adds multiple steps to what should be achievable within a few clicks. Avoiding the Print Module
Source: fstoppers.com