I'd like to create new content, based on some web form, but I need to use it like a folder in web client (especially navigate and add new subitems in many levels). I created a new demo type (extending wcm:avmcontent) and added child-association (targeted with my demo type), but I've no idea how to configure web client. Is it possible to browse associations in WCM web client in file/folder manner ?
Nope. This is currently not possible. When you do a deployment you'll lose of these anyway. Any association will have to be within the XML content file. WCM repository is different from DM. Although I believe these are being merged.