Yes, you can use a CSV file to format user import for CB Juice. You can then use CB Auto Actions to add them to groups automatically as believe CB Juice fires the after registration triggers.

Just cannot show those activities in the activity tab or module

You don't want to show this activity in CB Activity or it's not showing this activity like you're expecting? Please clarify. If you don't want the activity created for them then just disable the CB Activity plugin, do your import, enable it again.

I don't know how I can even create a subscription and insert users into a specific group instead of the default "registered" ACL group.

Automatically? CB Auto Actions can do both. Just setup new actions for CB Paid Subscriptions 3.x and Usergroup with after registration trigger. CB Juice I believe also lets you set the usergroup to give them on import.

About Activity, I would show them, but they are not recorded.

What activity is not recorded? Please be more specific. Does your CB Auto Actions work?

About user groups, Cb juice let me choose CB fields, not sure how can I choose multiple user groups to assign to my fake users.

I've no idea, please post your CB Juice specific questions to the below CB Juice forums.

www.joomlapolis.com/forum/226-cb-juice