Go to contacts then contact lists. You will then see a list of your existing contact lists. 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 three options how you wish to upload your appended list
Upload via a .csv file
Paste in via csv
Past in direct from Excel
Once you have chosen, click next. You will then need to match any custom data fields.
Next to email address select the matching email address field. Then 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.
Once you have finished matching up all your fields 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.