There are two way where you can Convert your PST file to MBOX file(i) Manual method (ii) by using third party tool. But the Manual Method is very difficult to use and its very time consuming method. So I would like to suggest you this Regain PST to MBOX Converter Software that is a Quickest Way to Convert Outlook PST file to MBOX file. User can also save 30 emails per folder at free of cost by using this Software free demo version.
For shifting, PST to MBOX migrator tool is one of the popular and safe solutions that allows converter of multiple PST files to MBOX. This application provides you quick conversion facility without any third-party tool support. The utility is very easy to handle without any difficulty.