You can add extra columns to any Row layer in your email by clicking 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 start populating with content layers (for example, text, images, buttons, etc) and customizing the layer's styles (for example, changing the background color, adjusting padding and spacing, etc) in Figma as needed.
Note that each Row can only have a single row of Columns, so you need to create a brand new Row whenever you decide to have a different number of columns as a underneath your existing row of columns.
ποΈ Column elements can only be added directly inside of π£ Row layers. If you need multiple rows of columns, you need to create brand new π£ Row for each row of columns you need.