I'm afraid that what you're looking for, is not how the Kentico tagging system works.
The built-in tags are created when you save a document with them. The Tag-field is parsed and each tag is either created or its usage number increased. I see a way of achieving what you're trying to do with a new and very simple module and the tags would even be automatically included in SmartSearch.
Let me first give you a few link to our documentation concerning Tags:
If you ran through these I think you'll agree with me, that the built-in tagging system doesn't work like you need. Here's a link to a page, which is a good start for creating custom modules: https://docs.kentico.com/display/K81/Creating+custom+modules
I can see how your module would have just one object type (Tag), and its own permissions, in order to let an administrator manage the tags. Creating the UI for such module is a piece of cake once you get the hang of it.
Now how to make it appear in the smart search? Well In your page-tape you'd have to include a field for the tags. It would be a UniSelector (preferably) allowing for multiple selection and returning the tag's 'displayname' or whatever column you think is the best suited. This approach has one inconvenience, and that is that if you delete a tag, it won't disappear from the document's properties but only won't be further available for tagging. However this is a very simple solution. Even compared to the one that's built-in. This way the tags are directly included in the page's information, tokenizeable, searchable, ... But you can't track their usage etc. But you could. If you further improve on what I've outlined here...