There is probably no way how newer EMLX files can be converted to older MBOX format. Both files contain email messages.
Convert Apple Mail email message to Mail mailbox.
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert emlx to mbox on OS X
The default e-mail client in Mac OS X
Convert from emlx file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.