Hi FN_68,
Thank you for your message in this forum.
According to your description, it seems that you want to import your notebook to your business account.
If so, we’re afraid that there is no out of box way to achieve that with business account. If you want to use this feature, you need to use Microsoft account, for more details, you can refer to this article:
Export and import OneNote notebooks. We’re sorry that it doesn’t meet your requirement.
We understand the situation and you make a great point. To address your concern about the situation, we’d suggest you kindly vote this UserVoice:
Add the ability to export and import OneNote notes and notebooks in Office365 for business.. This is the best platform to let us hear from you and make our products and services better for you and others.
For a workaround, you can use OneNote 2016 > File > Open > you can select notebook from the local. As shown below:

Then you can sync your notebooks to OneNote Online.
Thank you for your understanding!
Regards,
Sukie