It doesn seem easy to just directly convert iBooks ebooks (.iba) to standard ePUB format (.epub), which means your options for iba to epub conversion are somewhat limited. The only viable solution (more of workaround) would be to to export your .iba file as PDF and then import said PDF document to iWorks Pages. The program supports EPUB format, so it should be doable. The only loss will be in special formatting exclusive to iBooks.
Open IBA file Open EPUB file