You can add extra columns to any Row layer in your email by clicking on the Row layer in Figma, and then adding a new empty Column layer to it via the Quick Add Element dropdown selector at the bottom of the plugin window.

This will add a brand new empty Column which you can then start populating with content layers (eg. text, images, buttons, etc) and customizing the layer's styles (eg. changing the background color, adjusting padding and spacing, etc) in Figma as needed.