Hello Michele,
That would be a useful feature, and I have used other products that do have it.
ServiceNow cannot restrict access to a category with out of box functionality, it's probably possible to do so with an ACL on the category table.
The easiest way of restricting access to an article is by setting what role is required to see it (you may consider creating a role for this purpose).
A more complicated way would be to create a separate knowledge base to host those categories, you can then manage access to them from the KB level.
Once you have your new knowledge base with the permissions you need you can then move the categories following the steps below, but know that moving a category will not move the articles inside it, those will have to be moved as well in to the category in the new Knowledge base.
In SN here is how I do it: I've created a report that shows me the categories, make sure the "full category" is your first column it will make navigating this table easier
You can use the report as is or add it to a dashboard.
When you click on a category you will see the tick box where you can activate it or uncheck it to de-activate it (this will show\hide all articles within) but use this with caution as it may hamper with your KB workflow
If you click on the magnifying glass (twice) you will get to a popup that display a list of categories, here you can pick the one that will be the new parent category hence moving your category from one parent to another.
You can also move the category from one Knowledge Base to another by dropping down the "table name" value to Knowledge base.
I hope this helps.
