What exactly are you wanting to do? If you have existing ePub /PDF files outside of the app that you wish to read in the app, then you have a couple of options to side-load them.
1.Use iTunes. The information can be found
here
2. Have the file accessible on the device itself. For example; email attachment that can be read on the device. link to the file on a web page accessed from the device. In dropbox, with the dropbox app on the device.
In all of these scenarios, when you tap on the file you should see an "Open with..." option and Kobo should be presented as an option, and when selected the file will be opened in the Kobo app.
There was another thread where iOS file locations was being discussed, I believe someone wished to take a copy of the KePub files that were downloaded by the app.
As I mentioned there, I do know where the files are stored, however, unless DRM free, they will be encrypted, so even if you pull them from the file system, they'll be useless to you, as you won't be able to read them.