With regard to the SD Card File system, this is a very rigid file structure and it is not possible to just add files to the card using your PC.
The Top folder is designated as PRIVATE, as you have discovered, and then there are a series of sub-folders - TECHNICS > KN7000 > TFLD001 to TFLD099 (The actual 'Song' folders) and two other folders KN7000MN.INF (Which holds the actual Names of all the songs in the TFLD folders) and KN7000MN.BAK (Which is a backup of KN7000MN.INF). There may also be a CUSTOM folder in the KN7000 parent folder where any of your Custom Styles will be stored (XXXXXX.CST.)
There is a very good program which allows easy viewing and a good degree of manipulation of KN7000 files on an SD Card, using a PC. It runs on Windows (up to, and including Win. 10) and is called 'KN7000 SDExplorer'. It is available here : http://members.chello.nl/~f.mellink/SD-Explorer.htm
All Standard SD cards with a capacity up to 2GB can be used on the KN7000 and New SD Cards should be formatted in the KN7000 using the built-in facility > Press SD LOAD > SD Tools > Format. prior to saving files
There is also a very useful book for the KN7000 called 'Getting the Most from your KN7000' which I believe is still available Here : http://www.kn7000.com/product_detail.asp?ProdID=10701&SuppProd=&CatID=1
When the KN7000 was released it came with a CD called 'Applications and Drivers'. The programs were used to enable a direct Digital connection to a Windows PC, using a USB cable. The program - 'Song Manager' was used to transfer files directly but will only work with the versions of Windows which were around at the time - i.e. Windows 98 and Windows XP. There are ways to install and make the Song Manager work on later versions of Windows but they are quite involved. There are reports of procedures in this thread on SynthZone : http://www.synthzone.com/forum/ubbthreads.php/topics/378100/KN7000_USB_PC_Software
I have a small ACER Netbook with Windows XP installed, which sits on the music stand of my KN7000 and is only used for this purpose.