Hello,
As per your query to import PST files into Exchange Online Account. There are few manual solutions as well as you can use a third party software to move your PST file.
The Manual methods for the import process are as follows:
1. Drive Shipping
In this method, you have to copy your data into an encrypted Hard Drive and then physically send it to the Microsoft Data Centre.
2. Network Upload
First, upload the data files to a temporary storage location into Microsoft Cloud and later import those files into your account with the help of Office 365 import service.
@Dexter: You should try a third party utility if you wish to quickly perform this task.
So, another easy way to transfer PST files into Office 365 account is using a software named SysTools Office 365 Restore.
https://www.systoolsgroup.com/import-to-office-365/
Thanks & Regards,
Henry Davidson