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With "without further exposure," I mean without exposure that contains something you haven't 'seen' before. For example, reading an essay (not written by you) or watching a movie in that language is considered further exposure.
Having similar skills means that the other has more or less the same knowledge and fluency as you do.
It depends on your definition of improved. Writing and reading skills can almost certainly be improved without any further exposure in your L2. Although it might be helpful, it's not necessary to immerse yourself in television or novels written in your L2 in order to improve your grammar, understanding of syntax, or spelling. These are concepts that can be learnt without much difficulty from a grammar book in the language.
It is a more difficult task to better your speaking and understanding skills without further exposure, but it can be done. Generally, television provides a great way to practice your pronunciation skills and see how they compare to that of native speakers. Excluding further exposure eliminates this possibility, but if you know other speakers in your target language, and you (at least attempt to) converse with them regularly, you will begin to pick up the nuances of speech and pronunciation of that language from them. This was definitely help your listening skills as well -- if you ask for clarifications when you don't understand a phrase or word, you can easily increase the repertoire of words you are familiar with. At the same time, you will become more used to understanding different accents and dialects of the language.
In all, without further exposure, it is possible to improve your skills in your L2, but it is a difficult task to do so (especially to improve your speaking and listening skills if you don't have a willing language partner).
