To convert multiple EML files into a single PST file, you need to install an email migration tool that allows bulk conversions. I advised you to try SameTools EML to PST Converter. This tool converts numerous files at a time and stores them into a single PST file. It provides accurate conversions while preserving every email details such as formatting, metadata, content, attachments, etc authentically during the conversion. Every user with a Windows OS can install this tool on their system, as every version of Windows is compatible with it. The tool can be easily operated with just a few steps and no technical expertise is required.

For More Information: https://www.sametools.com/convert/eml/