@eyot. Can you remind us which version and flavour of ZorinOS you are using?
If you have Gnome flavour then I cant see how you can increase the nesting of apps beond the one level. However, others may know a way.
It may also be different on XFCE flavours of ZorinOS, but I cannot check that myself.
It is possible with XFCE.
Can you describe how?
Open terminal and issue the command:
menulibre
Well... yes, but how do you use Alacarte (Main Menu) or MenuLibre to create Sub-Categories?
Click on the plus sign (+) on the left top to add directory.
That adds a Category directory... But is there an option to cascade that under an existing directory to make it a sub category?
If you select any of the category before clicking the plus sign, it will make a subcategory there.
Got it; just successfully made one. I learned a new thing, today. 
Wow, it sounds so unreal.
I knew something you did not know?
As you said somewhere collected brains are better than one 
I am not so sure whether the OP @eyot is still with us, but I summarise how to create cascading/sub menu in XFCE desktop.
Open Menu Editor from the list of programmes or
from terminal (ctrl+ alt +T)
menulibre
Select the directory where you would like to create a sub menu and click on the plus (+) sign in a tool bar.
@Aravisian do you think you can tick this question as solved if OP never comes back to this thread? I think it would help for the future visitors who have a same question.
Nope.
I think I do not have a same privilege as you do.
This is what I see when clicking on the three dots under my post.
I made a mistake!
it should have been (ctrl + alt + T)
@zabadabadoo and @Aravisian
Can you edit the solution section if the original posting?
I do not want to misguide the beginners.
Sorry for the post & run guys
Shortly after posting this I received some family news that's knocked me sideways and I'm not really feeling it at the moment
Also I'd forgotten how good you guys were, I wasn't expecting replies this quickly
Would anyone mind if I put a pin in this & got back to it when I'm able to concentrate again ?
You are always welcome here to ask questions.
I am sure all volunteers in this forum are more than happy to help you out 
Maybe this can help you
Sadly, that Classic Menu has been deprecated and is no longer available.
But you have the right idea- sometimes installing a different menu can solve problems.
On XFCE, I use Whiskermenu in addition to the Zorin Menu.
On Cinnamon on Zorin, I use Cinnamenu on one monitor and Stark Menu on the other.
Hi all, I'm back, family issue is dealt with
So, thank you for all the replies & help
I'm running Zorin 15.3 Core version, the menu system is Gnome and not XFCE so it looks like cascading menus aren't going to be possible for me without a heroic effort that I really don't want to make
I think we can regard this as closed
