The manual methods to transfer your EML emails to MBOX are very confusing and very time consuming process for this you can try the
CubexSoft EML to MBOX Converter which makes the whole process very simple and smooth. It has works with all the Windows and Mac based systems. You can try the free trial to convert up to 25 emails from the mailbox for free. Also you can import into different file formats like MBOX, PST, PDF, TXT, CSV, NSF, Office 365, DOC, Gmail, HTML, and others. It comes with a very user friendly interface to help the both technical as well as non-technical users. This tool also supports batch migration and also shows live conversion and many other features which help in the migration process.