Can I add a tip to an appointment?

Yes, you sure can. What you will want to do is add a service under Setup > Services called "Tip." Set it to "For internal use only" so clients cannot book it. Then, during the Checkout process, you can "Add lines" and choose the "Tip" service, then manually enter the amount that the client gave you as a tip. When you save the transaction, this will also get synced to QuickBooks Online as a separate line item. If you are internally processing the payment via credit card, this tip amount will get added to the total charged to the client.