I want to add some text to the notification email that is send to the client. Is this possible?
Hi Jeroen,
Are we talking about the confirmation - cancellation emails for the Appointment type?
In that case, YES! That would be possible 
Please have a look at our guide here, listed in #9
https://help.collect.chat/article/show/88906-appointment-type
Kind regards,
Sibel from Collect.chat team