For the conversion of OST to PST, you can try OST to PST converter online Software which is an efficient way to migrate OST files to Outlook PST format. Through this third-party tool that smoothly recover corrupt OST files and also export several files into multiple file formats such as-PST, Zimbra, Live Exchange Server, Gmail, Office 365, MBOX, EML, EMLX, MSG, vCard, ICS, and etc. Along with it also show & manage all emails properties or mailbox attachments such as- Show the preview of all OST emails including these items: Messages Attachments Embedded Images HTML Formatting Attachments Header's Details Contacts Calendar Tasks Notes Journal and etc. The software supports all the version of MS Outlook. 

•    Download, install and run OST to PST Converter.
•    Click on the "Open" button to load OST file.
•    There are two options to browse OST file(s).

 Add File: To select OST file one by one.
 Add Folder: To select folder of OST files.

The steps to export the OST files to PST Outlook:
1.    Open Microsoft Outlook.
2.    Click File menu and select 'Open & Export' and click on Import/Export from the available options.
3.    From the Import and Export, click Export to a file and select the file format option – PST from the suggested storage options.

Click here for more info:- https://www.kdetools.com/how-to/convert-ost-to-pst-without-outlook.html

With the help of Email Backup Wizard, users can import Gmail emails to another Gmail account easily without any complicated steps. The software create backup of Gmail emails, calendar, attachments, etc. to another account with all the properties.

Features of Email Backup Wizard

• Creates backup of emails in an easy manner
• Import Gmail emails with all the attached files
• No file size limitations to take a backup of Gmail emails
• Takes backup of selected emails with the filter option
• Compatible with all the latest and below versions of Windows

Read more: https://gallery.technet.microsoft.com/save-gmail-as-gmail-02d63364