Thanks for posting in the Community. I'm here to assist you with copying data from one company file to another.
You can import the lists and transactions using an IIF file in QuickBooks Desktop. However, you're only able to import supported data using this feature.
For more information about using IIF files, you can visit this article: IIF Overview: import kit, sample files, and headers.
If you want to export and import MS Excel files into your QuickBooks Company file, you can also check the outlined steps in this article:
Since you need to import everything from the other company file, you can also check for any third party app with the process. You can try searching them through our Intuit Marketplace website.
Please don't hesitate to reach out if you have further questions concerning QuickBooks Desktop. The Community team is always here whenever you need assistance.