Can user after choosing value in cb_interests field automaticcaly added to group like in action after registration?
Yes that's possible. You'd act on the after registration trigger with a usergroup action and condition against cb_interests under conditions.
Is it possible also make action to add user in linked group after set cb_interests value in back-end admin?
Backend has direct access to the usergroups field. You'd just change the usergroup the same time you change cb_interests so I typically do not advice configuring an auto action to do this, but yes it's possible using the after new user trigger.
See the below videos to get started with CB Auto Actions.
www.youtube.com/playlist?list=PLp0puRITgC7McQcw0Sli1qfSUQGwmflzi