Beyond Dawlish

Importing PST files to Shared mailboxes is not possible manually, as there is no direct method to do so. Additionally, there is no native tool provided by Microsoft for that. But, you don't need to worry as you can opt for automated tools. One of the most reliable and preferred solutions is SysTools Import PST to Microsoft 365 Solution. This utility comes with a simple UI and several exceptional features. 

Try this solution for free, and import all the PST files to the Exchange Online in minimum time!