Thanks for posting! I took a look at the February email and see the column you are trying to add. Unfortunately it is best to pick a template with the layout you are looking for rather than redesign the template once you've started working with it. I did notice the second email "template update" that looks like it has a split layout. Will this work?
If you are really wanting the left column layout it would be best to go back to our older editor. I see you have some emails in this account that used that version of the editor, would you be interested in going back to this? If so you will lose access to the new editor but could copy and re-use any template that was created in the new experience.