You can choose to not import your client list into Full Slate and just let clients populate your list for you as they book appointments. However, the disadvantage of this is that you won't have client information in the system when you go to book appointments yourself (for example, when clients call in or book after their session).
So an approach you might consider is to import your clients (check the "How do I import my client list?" for detailed instructions) and then invite them to register for a Full Slate account. That way you can get your clients' information into the system and yet be assured you won't have many duplicate client records once clients begin to book.
You can invite clients to register by sending them the 'Invite client to register' email template. You can do this for individual clients under Clients > Details (click 'Send message' and select the 'Invite client to register' template) or in bulk under Clients > List.