Actually, I downloaded the metadata for each book and edited that specifically independently.
For others, this is what I recommend: I created a CUSTOM column which I called "Collections". That was used specifically to mirror to the desired Collections on the Kindle. Then I was able to use multiple words (descriptions) within the tags column, or in cases leave that as the metadata was from downloading that (from others such as Google, Amazon, Open, etc.).
Yeah, it was a lot of work, and really wouldn't have mattered when I started.
NOW, for simplicity, I wait until I have 20+ new books and do the above in a small chunk which takes me less than an hour.