Unfortunately, as you discovered, there is no way within LR to edit your keyword hierarchy outside of LR and then import the changes. It was an oft-requested feature in years gone past.
If you try to import your CSV file, you'll end up with a brand new set of keywords, and your existing hierarchy won't be changed.
You can edit your keyword hierarchy within LR in the Keyword List pane. E.g. you can create the top-level Animal keyword, then drag the Dog keyword onto it, then create German Shepard and Poodle under Dog. Photos that previously had Dog will retain it, but it's now Animal > Dog.
I invented the Keyword Consolidator plugin (a long time ago) for restructuring (and consolidating) keywords, but it's kindofabitch to use, still it may be worth a go (it's what I used way back when I had a big mess, now I only have a small mess :-}). I confess it was a painful and time consuming process, but it helped me get the job done, and it's free:
Anyway, if I were doing it over again, I'd do it differently. In fact I consulted with somebody who was planning to code what would probably be a very-nice keyword consolidation plugin, but I haven't seen it finished, and I don't even remember who it was now.
Anyway, if your keyword spreadsheet includes the information for which old keyword(s) should be mapped to the new keywords, it would be fairly easy to make a little script which migrates everything (creates the new keywords, and replaces the old with the new for each photo).