PhoneBurner features functionality that allows you to append an existing contact, it's called Contact Append.
1. Click My Account > My Account in the top right hand corner.
2. Select the last link, Advanced Settings at the very bottom of the page
3. Review the Contact append feature, turn it to on, and make sure you save.
With this feature enabled, the import tool allows you to update existing contacts. The next time you export your contacts, a new "Contact ID" column will be included. Make changes to the spreadsheet and import it them as you would normally do. Map the "Contact ID" column and the contacts are updated using the data from the spreadsheet. Appending is optional and only activated when the "Contact ID" column is mapped - allowing you to import data like usual.
**NOTE: When you are doing an update and map phone numbers, we will add those phone numbers to the contact instead of updating existing ones.**
**NOTE: This must be done by export and then update , cannot be done by just adding a list, this will create duplicates.**