Assuming you are referring to "users" in the back end, JobRoller uses the native WordPress user functionality, so all users that are created appear on this page. This is how users are managed by default in the WordPress admin dashboard (not for just JobRoller), so any change to the way this works would require customization on your part.
Our themes make use of WordPress custom post types, and "Jobs" are separated from "Posts" because posts relate to entries in the blog which are separate from job listings. Blog posts appear under their own "Blog" section as opposed to the actual job listings.
JobRoller is not encrypted, so yes you are free to make modifications if you wish.
Job Packs can be manually assigned under the users name in the "User" function in the WordPress admin dashboard, and there is no specific limit to the number you can assign. Thanks.