If you have an existing Hilton Honors membership number, you will be prompted to enter this when activating your card via the Hilton Honors Debit Card app. We forward this data to Hilton Honors who should be able to match it to your existing account as long as the email address and name you've signed up with matches those details in your Hilton Honors account.
If they can't, they may create another Hilton Honors membership number for you. If this happens, please contact them directly so they can merge the accounts in their system.