Yes, you can use an existing WhatsApp number with OWA. However, you will need to first delete the account linked to that, prior to onboarding with Interakt.
If number is being used by another Business API, you still can get it migrated to OWA after getting the 2 Factor Authorization (2FA) pin disabled from your current BSP. Or, you can just provide a new phone number on which you can receive an OTP - this could be a virtual number or a regular SIM number.
Once migrated to WhatsApp Business API and linked to OWA, the number can be accessed on OWA only.