I'm a bit irritated: Your profile reads "Nook", this is the "Kindle developer's corner" and your question seems to be about running Koreader on a Kobo (?). As for running Koreader on a Kobo, see this thread:
https://www.mobileread.com/forums/sho...d.php?t=216960 - but a short answer: I'm not aware of a mechanism that plugs into Kobo's document choser, you would have to resort to Koreader's file choser.