For books downloaded from Kobo, where they are stored is handled automatically. If the main memory starts to run out, and there is an SD card in it, the books will be put on the card. This starts happening with somewhere between 50MB and 100MB free. If you are sideloading the books, you decide which memory to put them in.
If you really want to do this, you can put a file on the main memory to fill the space, sync to the server to download the books to the SD card and then remove the file. I have done that as a test and it works.
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