Quite possible.
Do the following:
Find an article that you want to belong to the new "Inferno" category, click edit, and add the following line:
[[Category:Inferno]]
Then click on the category, and add a line to it that says something like this: "This category is for Inferno-related articles." Adding any line will actually create the category. If you don't do that, the category will be colored red even if there were dozens of articles put into it.
Now, add all the relevant articles into this new category.
To make Inferno Ships a subcategory of Inferno, go to the article Category:Inferno Ships, and add the Inferno category to it as if it were a normal article.
I suggest you make the Inferno category a subcategory of some other category, such as Category:User-made Campaigns. It will make the main Inferno category easier to find.
Also note that there is no h in category.