I installed kmail with imap and forgot to say, that I want to be able to work with kmail if there is no internet connection. Is it possible to change this additional?

Go to your account configuration and tick "Make all messages available for offline use". Like this, you can view them even if you don't have an active connection.

Thanks a lot! I overlooked it. And yet another question: Is there a special place where my emails are saved?

They are saved, normally, under ~/.local/share/akonadi/file_db_data (one file per mail).