Yes, you can do this. It's especially useful when you're using automatic confirmations and you want to give clients a choice of pre-payment (e.g. if someone wants to purchase your service as a gift). This way you can take advantage of a number of features of checkin24-7 which track PayPal payments. This way in reservation details you will always see if there is a payment associated with the reservation. Also, if you use checkin24-7 to create invoices, the amount paid/owed will be properly displayed.
You have two choices (you can use both methods).
1. You can create a customized message sent to user upon reservation with the $(payment_form_url) tag in the body. This tag will be replaced with a link to a payment page on your own website (when advanced integration is used) or on checkin24-7.com (in case of simple integration).
2. You can also change the reservation done template (in settings/templates) so it displays the payment form even though the reservation is confirmed. You should use the $(payment_form) tag - it will be replaced by checkin24-7 with the code for the payment button.