Could I register our existing number of WhatsApp Messenger App or WhatsApp Business App to official WhatsApp Business API?
Yes, but you need to delete all the related WhatsApp services to this number before submitting API registration.
To delete your account
1. Open WhatsApp.
2. Tap More options > Settings > Account > Delete my account.
3. Enter your phone number in full international format and tap DELETE MY ACCOUNT.
4. Select a reason for why you're deleting your account in the dropdown.
5. Tap DELETE MY ACCOUNT.
For details on account deletion, please refer here.