When it comes about MBOX files conversion into any other popular format, I recommend SysInfoTools MBOX to PST Converter
tool. It is an advanced way to convert your MBOX files into PST and other file formats, such as: EML, MSG, RTF and HTML. It can easily convert an MBOX file which is created in any email client, such as: Thunderbird, Entourage, Spicebird, Pocomail, AppleMail, Netscape, Incredimail, ClawsMail, OperaMail, MozillaMail, Evolution, Sylpheed, Eudora, Mulberry, SeaMonkey, etc. It does not cause any damage or data loss to the original MBOX files. In fact it creates new file and saves all the data from the selected MBOX file.
You can read more about this here: www.sysinfotools.com/mbox-to-pst-converter.html