For now, you can't apply newly added source rules to already received 'Unable to determine' alert.
The options you have:
- Open the undetermined alert, select the second option and set up all the settings again
- If you already know the person or a group who's responsible for this alert, you can acknowledge it first and then assign to a specific user (user will be assigned and notified) or group (alert will go through group notification policy).
If you use this variant you can perform bulk change for a number of undetermined alerts (check-boxes on Alerts tab)