For example, if I wanted to add another path to Czechoslovakia, would it be possible without manually adding each focus from the death or dishonor DLC? 

Of course! You will need to create a new focus tree on the site, then import the Czech focus tree and localisation file using the "Import" option in the sidebar of the focus tree tool.

From there, you can add to/edit/delete parts of the focus tree as you see fit.