|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.