Something that the answers above did not cover is the very strong assumption made in the title regarding the scope of the word of God, which is important for the question itself. Established Chalcedonian Churches would not make any claim of the form "all of the word of God is contained in..." or "there can be no more". Indeed, the two most prominent Churches, Catholic and Orthodox explicitly reject that view. For a very recent and clear explication of the Catholic view on the matter, see the Verbum Domini here: http://www.vatican.va/holy_father/benedict_xvi/apost_exhortations/documents/hf_ben-xvi_exh_20100930_verbum-domini_en.html
Word of God, strictly (and capitalised) refers to Christ, an eternal an inseparable part of the Trinity, as established in John. Naturally, there is no sense in which the Bible can "fully contain" Christ, and a fundamental precept of the Christian fate is that the Word is eternal, not only can "there be more", but there is. The Christian tradition, explicated in human words can be referred to as "word of God", and frequently is referred to in such a way, but the reference, for orthodox traditions, is one by analogy and to signify that particular human words announce, contain or explain the Divine truth. It's worth noting that "theology" is, literally, "word of God", and that was the only meaning of theology in the Christian world up until the time of the scholastics, when it started acquiring the more modern meaning of a rational study of religious concepts — and scholastics certainly would have found the statement "there can be no more theology" very strange! Generally, by orthodox doctrine, the words which explicate the Divine truth are not limited to Scripture, the exhortation linked above is clear on the topic: "the word of God precedes and exceeds sacred Scripture". This, of course, is not to detract from the singular status that Scripture has within the orthodox traditions.
This should help you contextualize the use of the Nicene Creed, which, as correctly answered above, is not a supplement to the Scripture, but a concise and clear re-statement of the truths already contained within the Scripture. Why is there a space for such re-statements? Exactly because the word of God is a living tradition, a tradition which ensures our continuing comprehension of the Holy message expressed in the Scripture. And here 'expressed' is a far better word than 'contained' — orthodox traditions do not claim divine authorship for the Gospels, they are human works comprising human words, for all words are human — it is the divine inspiration behind the words that gives Scripture its singular status.