When you backup calendar data (.ics) in Apple Calendar (iCal), an .icbu file will be created that stores the backup. This is probably the only way how to achieve this ics to icbu conversion, although it's more of a standard backup process than conversion.
Convert iCalendar calendar file format (Microsoft Outlook, Apple iCal) to Apple iCal backup.
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert ics to icbu on OS X
Apple's personal calendar application for Mac OS X
Convert from ics 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.