How do you add more columns to a sub-table that has existing columns?

How do you add more columns to a sub-table that has existing columns? I have been searching for this for days, both on the Ninox site, and in the app. I tried right=clicking, left-clicking, red wrench icon on/off, no luck. I sure miss Bento. This app is not very intuitive, and has a very strange design, but once you learn some of the more quirky steps, it works. Sometimes though, no matter how hard I try, it is just impossible to find something. Any tips for adding a column?