I suggest Mail Extractor Pro for the conversion of Apple Mail to PST. The tool is known for it's in-built file splitter, auto-load of files, safe and secure conversion and many more.
Visit here: https://www.uslsoftware.com/apple-mail-to-pst-converter-for-mac/

Gain More Features Facility by using  'KDETools Batch OST to MBOX Converter' software that easily converts OST emails, mailbox, Contacts, Calendar items, and etc. into MBOX ( Thunderbird) along with it also works multiple Emails, clients, such as- Mozilla Thunderbird, Mac/Apple Mail, PocoMail etc. This software is a great conversion app, designed for precise export of multiple OST files to MBOX and also save data into multiple file formats such as- PST, Gmail, Zimbra, MSG, EML, EMLX, vCard, ICS, PDF, OLM, NSF, Office 365, Live Exchange and etc.
More Info- https://www.kdetools.com/ost/mbox

Rapidly works & instantly create corrupt OST files to MBOX file with safely all data of OST files by instantly great technology name of OST to MBOX converter software.

Advance Benefits-
* Convert OST to MBOX
* export corrupt files from OST to MBOX with Outlook
* Export Calendar to ICS with Zimbra/Live Exchange/ MSG/EML/EMLX/OLM/NSF/MBOX/ HTML and etc.
* Migrate OST to Office 365 with live Exchange server
* Batch Option- easily select multiple files from OST to Outlook
* split large PST file ( 25GB)
Filter message by "Date"
* Software easily Conversion all items to PST without installing MS Outlook version

More Info-: https://www.kdetools.com/ost/mbox/

You can use the Email converter tool for converting email files into other email files format but before using any tool you should know the complete knowledge about why we need conversion of email files? Here I share article link:  Manual conversion of Email files  from where you will learn all the details about popular email files and the best conversion methods for OST to PST, EDB to PST, NSF to PST, OLM to PST, MBOX to PST, EML to PST, DBX to PST, DXL to PST and PST to EML files format.
 

Advance Solution for convert OST to PST
If you are looking for the best way to convert OST to PST, then you need the best OST to PST Converter software solution to convert OST to PST. It Convert OST to PST very easily and gives you accessibility over corrupted or orphaned OST files through OST to PST conversion. The software can allow user to save the recovered email items in PST, MSG, EML, HTML, MHTML, and RTF.

More information visit: http://www.osttopstsolution.com/microsoft/

The best option here would be to converts the .ost backups to .pst,

I once tried KdeTools for OST to PST software and it was quite helpful. The ost2pst software helped you in restoring corrupt all OST data and converts OST to PST file format in just a few minutes along with all items. 

http://ost2pst-converter-free.software.informer.com/

Hope it'll helpful for this suggestion ...!