The lists of Suppliers and Customers import your Contacts from QuickBooks Online.
You can add new Contacts to the list in QuickBooks Online. After you have added the new contacts, click on Integration in the Soldo web console, then click on the Refresh button in the top right hand corner of the QuickBooks Online Integration. It may take a couple of minutes for the list to finish refreshing. The process is virtually identical for adding new suppliers.