When first creating an email template, you have to option to choose how many columns you would like your email to have. However, once you've already started working on a template, you may decide that you would like to add additional columns. To do this, simply drag an Add Columns to Email block into your template.
Once you've dragged additional columns into your email, you will be able to edit each column separately. This means you can add borders, change the background color, padding, etc. of each individual block within a column. You can do this by selecting the block and navigating to the Block Styles tab. Each column will pre-populate with a text block, but you are free to change this to a different type of block once the columns are added.