Adding values to formdata of ModelForm before saving

Is it possible to put missing data into the data of a form before calling form.save() ?
I want to add the calendar-id (calid/in_calendar) before saving to the model.

 http://susepaste.org/23451355

I tried adding it after instantiating the form with form.in_calendar_id = kalender but it didn't work
I don't want to use hidden fields because manipulating them is easy

Maybe https://groups.google.com/forum/?fromgroups#!topic/django-users/F5yHH-G5QLM describes how to solve it but I didn't understand it.

PS: If you find very ugly code please correct me -- I'm a beginner

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