The lists of Suppliers and Customers import your Contacts from Xero.
If you wish to add new Contacts to the list this should be done in Xero and then the integration will need to be refreshed. After you have added the new contacts in Xero, click on Integration in Xero, then click on Refresh button in the top right hand corner of the Xero Integration. This will refresh the list. It may take a couple of minutes for list to finish refreshing.