In the shapes toolbar you will see categories (different ones
depending on which type of template you chose to open) and if you
right-click on the category (ex. Basic shapes) you can close it. But
there is no way to add in categories.
Ah, I've suddenly realised what you are talking abou!
I though "shapes toolbar?", there is no shapes toolbar.
What you mean is the actual stencils themselves!
When creating certain drawing types a number of stencils are also opened. I presume you
have been closing some of these. You can add any new stencils, use the menu File -> Shapes
and explore all the extra shapes you can use.
If you want to find out what all the stencils were when you created your diagram then just
create a brand new one, write down the names of the stencils and go and find them and open
them as per the above instructions with the new diagram.