KOReader has several functions for this. Its "Cloud storage" lets you setup for "Dropbox" or "FTP". There are two ways to access your calibre library wirelessly, the easiest is to just turn on the calibre content server and access the OPDS local calibre catalog. You can also have your KOReader equipped device appear in calibre as a connected wireless device, using KOReader's "
calibre wireless connection".
You can, of course, place the downloaded books where you want them and read with any compatible reader on your device, to include the stock software.
Luck;
Ken