Association and a class join are the same. Associations are provided to let users without designer studio get access to class joins. Developers are responsible to find all the required class joins that end-users tend to use and create associations for them so that they can use them in report browser directly.
To put it simply, no we can't create associations from report browser.
Could you please let me know how to add(not create) an association to a report in report browser (like in a screenshot), I am checking all the options in report browser but couldn't find the association that i created in developer portal.