you described much better but as a tool I suggest MailsDaddy MBOX to PST converter, which is the best software to migrate MBOX data to Outlook PST format. The tool has inbuilt lots of features and it supports 30 email client's mbox data such as Mozilla Thunderbird, Apple Mail, SeaMonkey, Spicebird, Netscape, Evolution etc. With this tool, you can convert multiple mbox to pst and other given formats. The tool has been already tested with huge MBOX data so don't worry about huge MBOX data.