[Roy Dennis]

I would think that if you tried copying a folder of kma files in windows explorer to your kma folder, normally windows will bring up a dialog with 3 options in it if it finds a duplicate named file.
Copt & Replace
Don't Copy
Copy but keep both files
There is also a tick box at the bottom, "Do this for the next xxx conflicts"

If you tick that box and select the "Don't Copy" option then it will only copy the ones that don't match.

The trouble is those kma files will all have a book id allocated to them that is already used in your own kma files database, as soon as Hoster finds these it will probably tell you you have xxx duplicated files even though they are not duplicates by disc id or title artist but just because the book id are the same.
You will probably get an offer to merge them which will renumber all your book id's so if your not bothered about it renumbering all your songs you may get away with it. If you have your song books numbered with book id then they will probably all be wrong and need printing again.
If I was going to try this I think I would make a backup copy of all my song files first on a spare hard drive so that I could revert back to it if things go wrong.