Hey @Robinn, thanks for sharing this!
It isn't possible to add another menu line with Square Online site at the moment. However, you should be able to create a drop-down menu for your categories!
To do so, navigate to your Square Online site overview > Website > Edit site. From here, click into Header and then Navigation.
Within this, you'll be able to drag and drop navigation links to make sub-links OR if you hover over each links, you'll see a + button to create sub-navigational links as well.
I hope this will help.
