You can sideload books where ever you want as long as they are not in a directory starting with a ".". So, don't put them in the ".kobo" directory. There will also be a directory called "Digital Editions". This is where ADE will put books if you use it. Sideloading books there will probably work, but it isn't advisable.
My recommendation is to create a directory on the device for your books. As you are borrowing a device, you could name that to know they are your books. Alternatively, use calibre and let it handle where the books are.
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