How to add a new case without completing other information?

Here are my answers to your questions
1. You can freely add existing case files to the system without completing all the worksheets.
Step 1. +New Case
Step 2. Choose Case Type, Subtype and type in client information (you can put a virtual/fake email if the client does not have a contact email)
Step 3. Do not select 'Share case with client(s)' and Save
Once you have accomplished these 4 steps, the case file will be created in the system. You can leave it as it is if no further steps are required.