iBooks Author supports import of ePUB ebooks since version released in October 2014. Such ebooks can be used to create a new iBooks ebook, which is first saved as .iba then can be exported and published as .ibooks.
Convert Open Publication structure eBook to iBooks e-book file.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert epub to ibook on OS X
Apple's free authoring tool for making great looking e-books for iPads
Convert from epub file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.