Appointment Booking Pro costum work

I would like to do some custom work in the system called : Appointment Booking Pro [login to view URL] The changes I would like to make is mostly in the file called getSlots.php. The file shows the timeslots you can book. The following should not be displayed in simple booking form: - Timeslots that is taken - Timeslots that uses time after end of business (If end of business is at 15:30 and the appointment is 40 minutes you should not be allowed to book at 15:00) And also I would like to: - I would like to bee able to combine 2 or more services (accumulate the time needed) All should work inn the simple booking form.