Even if you do "kepubify" an epub from elsewhere, it is not going to help you at all.
For things such as Pulse, Bookmark Sync, Highlight and Note Sync, the book *has* to come from Kobo. Side loaded content simply will not work with these features, well, at least not on iOS.
One of the reasons being is that we use a books unique id, and that is generated when the book is added to our system.
Additionally, the epub from somewhere else might not be exactly the same file as Kobo provides. perhaps there is an additional page in there, and this difference in actual content of the file could cause the features to become non-functional.
There would be substantial development required, by at the very least, the iOS team, and possible other teams at Kobo to implement these features for side loaded content.