The best way to convert MBOX files to PST format is to use Softaken MBOX to PST Converter, which easily converts MBOX files to PST format. I use this application because of its ease of use. It is a useful software with an easy installation process. It supports various email clients like Thunderbird, Apple Mail, and SeaMonkey and you can also use its demo version. You can convert some MBOX files to PST format using its sample.