It's simple and easy to convert PST into MBOX. I recommend using eSoftTools Outlook PST MBOX File Converter (https://www.esofttools.com/pst-to-mbox-converter.html) software. This software can easily be used and all Outlook PST files can be converted to MBOX files. Split options are also provided to all users to divide the huge files into small multiple files based on the size. It can offer a free demo to all users to convert initial 25 items from each pst file to mbox. 
Read more:- Convert PST to MBOX For Free