You can use
KOReader's OPDS support to view site catalogs for the ebooks and download them wirelessly, including your local calibre media server catalog. KOReader also can be recognized as a wireless device in calibre (like calibre companion on Android).
There is also
new Cloud Storage & FTP support. As well as the zsync support, for transferring between devices running KOReader.
The OPDS sites that come with KOReader include; Project Gutenberg, Feedbooks, ManyBooks, Internet Archive, as well as the Local calibre catalog. You can also add your own, with the "Add new OPDS catalog" selection.
Luck;
Ken
PS; Once you have downloaded a book using KOReader you can read it using the stock reading software (assuming it is in a format that Nickel can use).
Also: the latest KOReader download site is
here.