add subcategories to JOOMLA

hello if you know joomla , you will know it's only support this structure SECTION >> CATEGORY >> ARTICLE and you can't add any more sub-level categories what i want is the ability to add subcategory where the subcategory will appear under the parent category in the front end i also want to be able to add articles to multiple categories from the back end "not very important" the most important thing is ability to add unlimited subcategories as needed thank you