Hey! If you're just looking for a simple and free tool to convert EML to MBOX, you might want to try Aid4Mail MBOX Converter (Free Edition) or Emailchemy (limited free trial). There's also Free EML to MBOX Converter from Softaken basic but works for batch conversions.
For open-source options, Maildir2Mbox (Python script) or eml-to-mbox.js on GitHub are decent if you're okay with a little tech setup.