I'm sorry for writing too much, but this is my first site with joomla and I'm a huge noob!
In the schedule page I've some hours (00:00 - 2:00, 2:00 - 4:00...) but sometimes a dj wuold like to trasmit at 5:00 to 6:00 and another at 4:00 to 5:00, so I'd like to add some extra field (more than 12), is it possible?

Sure,

Open K2's extrafield view.
You will notice that almost all these fields refer to the schedule.

What you need to do is to copy exactly (and I mean exactly) a full set.

eg:

22:00 - 00:00
Alias: Header_m
Time
Alias: time_m
Avatar
Alias: avatar_l
Title
Alias: title_l
Tagline
Alias: sc_tagline_m
Cover Image
Alias: coverImage_m
Dj
Alias: dj_m


Be careful to use the same patter in aliases. Eg: the next in line should have an alias of Header_n .
This way you can create an hourly schedule.

Finally, you do not need to edit any code in K2's files this pattern takes care of that.