Generally speaking, no.
That's because the Kobo books will have their own version of Digital Rights Management (DRM) on them (as iBooks have on them). Each manufacturer/publisher have their own DRM system, so no book bought for one will work on the other.
This is, of course, if the books have DRM. If they are DRM-free, then you can transfer them between book apps.
You can always try to read them with the iBooks app - but if it fails, that's why.
Marilyn