The most suitable free EML to PST converter software depends on how the tool handles the conversion of EML files, including whether it supports bulk conversion. I recently used the WholeClear to PST converter, which supports batch EML to PST conversion. So you can also try converting your EML emails from Thunderbird, Windows Live Mail, or Outlook. It maintains original attachments, email format, metadata, and folder structure. In addition to that, it retains details such as sender, recipient, CC, BCC, subject, and date. In accordance with its product page, Outlook is not required to be installed in order to do the conversion.
Before doing any conversion, you can try the free version in case you plan to process a large mailbox, as the free download usually provides only limited options.

For bulk conversion of MBOX to EML format files, you should choose a dedicated and capable tool for processing multiple MBOX files at once. It would be easier than converting individual e-mail messages separately.
The TrustVare MBOX to EML Converter can perform batch conversion without the need to install either Thunderbird or Apple Mail.
Here are the instructions:

1. Select File Mode to load certain MBOX files to process several MBOX files within one folder.
2. Check the imported emails and make sure that the right MBOX file data is selected.
3. Set up the desired format (EML in this case).
4. Choose a location for a folder to save the processed emails to.
5. Start conversion by clicking Convert Now and then analyze the log.
   In order to ensure that all information has been preserved, open some EML files and see their sender and recipient info, timestamps, content of the message, and attachments.

You can import your EML files into Thunderbird. You can try the manual method for importing, but it's too difficult when you manage multiple large files because this method consumes a lot of time and risks data loss. You need to try the advanced tool. The DataVare EML to MBOX Converter helps to make the process easy. This tool helps to import multiple EML files into Thunderbird; it may be easier to convert them from EML to MBOX prior to moving them to Thunderbird. The utility for converting EML files to MBOX allows you to batch convert your EML files. Once the conversion is done, add/import the MBOX file to Thunderbird and make sure that the emails, attachments, and folders are there. Remember to create backups of your original EML files.

To import the OST file into Outlook PST format, you need to choose the best OST-to-PST converter tool, because there is no direct way provided by Outlook to open and convert the orphan OST file to PST. The GainTools OST to PST Converter helps users to select the required OST file; the preview option is used to check the data within it, and it converts the entire mailbox. The OST Converter will save the converted PST file at the chosen path while preserving the original structure of the mailbox folder. This tool can be used in case the OST file belongs to an inactive Exchange mailbox. It can scan the required OST file, display the data from the mailbox, and convert emails, contacts, calendars, tasks, notes, and other folders into PST.