Go to lists to view your existing contact lists. Click on the settings icon to view the different list options.
The append button allows you to add a list of additional email addresses to an existing contact list, which will be de-duplicated for you upon upload.
You will now be presented with the list upload page. Choose from one of the options how you wish to upload your appended list.
Once you have chosen and uploaded your data, click next.
You will then need to match up your email address column.
You will then need to match any custom data fields.
Map any columns for personlisation and list segmentation as you would with a normal list upload.
Notice that you’re only able to map out fields that are present in the existing list.
Click next. You can then add an email address to the box of where you would like the confirmation email to be sent to. Click finish and your list will be appended.