Reading Caesar II user manual in the chapter regarding Steel Modeler it seems that it's possible to insert data using a command line. Is it my misunderstanding or it's really possible??
If yes how can I do it?

A command line - "no". A "text file" that you construct using an Editor - "yes".

The "text" file that you would build with your Editor is the ".str" file.

But be careful! I did the same and then found out that this file type comes and goes... When I built a structure using a '.str' input file it actually worked. Then I also used it to edit my model and that DID NOT work. I cannot tell the exact details, but it looks like CII converts the .str file into a binary .C2S file after closing the session and deletes the .str file. When a new session is opened a new .str file is written. I found this out because I kept the input file open with my editor and after restarting CII my editor suddenly reports 'File has changed, do you want to reload?'

Yes, that is exactly the way things work. A "structural job's" files are zipped into a "c2s" archive while a "piping job's" files are zipped into a "c 2" archive. This was done for three reasons:

1) Some users are intimidated by the number of files associated with a job.
2) The archive takes less space on the hard drive.
3) The archive makes it easier to move the job (either for archiving or support purposes).