I would like to suggest you to use of 3rd party software as well known Advanced MBOX to PST Conversion software which is capable of converting MBOX to PST files. Read more and free download this software, click here
You can try the new and best MBOX Export tool that helps converting MBOX files of any size and from any application like Thunderbird, Apple Mac Mail, Entourage, Eudora, Netscape, Seamonkey, Spicebird etc to Outlook PST Files. With that the Software offers few more Output foormats like PDF, Lotus Notes NSF File, EML , MSG etc to convert.
Knbow more detail :>- http://www.filesrepairtool.com/mbox-to-pst-converter.html