P: In the Export dialog, provide the option to add a simple borders

You can use Borders plugin from LightroomToolbox:
https://lightroomtoolbox.minsky.com.ar/en/bordes/

It lets you to add simple and complex frames. There are three types of frames:

• internal: they partially cover your photo (you define the border thickness, color, transparency)
• external: arround your photo (you define the color, transparency and thickness in any direction)
• frame: they are around your photo. it is used to complete a defined aspect ratio for instance if you like to export photos with different aspect ratio and want to get all exported files with a 4:5 aspect ratio or whatever you want. (you define the aspect ratio, color, minimum thickness and whether the photo will be centered)

• All those types can be combined and used together
• external and frame borders can use you photo for its definition. The photo is expanded to cover the border, and you chooso a blur, desaturatioin and transparency that will be used. This creates a border that combines with your photo
• All those types have inner and outer contourns you can define
• You can also add shadows in order to create volume (for instance a countour representing the wooden part of the frame project shadow so you get volume)

"Export" please allow us to add borders. (Feature request!)

Meanwhile, may be this plugin can help (I have never tried it.)
Export plugin "LR/Mogrify 2": https://www.photographers-toolbox.com/products/lrmogrify2.php

You might be interested in the LR/Mogrify plugin which "... allows you to add outer and inner borders, watermarks and text annotations to your images." The plugin is available as donationware here. Read the Quickquide for a description of its features.

In the Print module you can add a stroke border to the image and then print (export) to a JPG file.  In the Export dialog you can modify the image sizing.  It would be wonderful if you could add stroke border to the Export options and add resizing to the Print module.

Currently you have to print to JPG with a stroke border then Import back into lightroom.  Then Export and select the resizing options. It seems like you should be able to do both in either module.