Can I enter in the Console appointments with clients that happened before opening my Console account? I need those appointments in my Certification Report.

Yes, you would do this just like you would schedule future appointments. Click on the Calendar menu, then click on the button '+ Create new' in the top right corner (system version 4), respectively the 'New Appointment' button in the top left corner (system version 3). You can select a date for the appointment in the scheduling window. To select past months/years, you'll use the little  calendar arrows on the left side of the month/year to navigate, select the date & time of the past appointments, add the client/group, then save.