According to this thread http://judaism.stackexchange.com/questions/13447/is-it-a-discouraged-or-an-unacceptable-practice-for-a-gentile-to-keep-the-sabbat?lq=1 It seems that non jews aren't allowed to keep shabbos. Doesn't that make this an eighth mitzvah (in addition to the 7 Mitzvot being Noah) ?
There is a discussion as to whether a non-Jew observing Shabbos, or studying certain parts of Torah is included within the prohibition of theft or sexual license (e.g. Maharsha, Sanhedrin 58b). The real debate is whether the relationships between the Jews and Shabbos and Torah is more like property or like marriage. But in either case, the connection to the Jewish People is considered similar enough to something already prohibited to be included in that umbrella.
Which touches on a more fundamental point; the 7 mitzvos are pretty broad. There are counts of 30 (Chullin 92) or 66 mitzvos that are described as subcategories of the 7. The count of 30 is explained this way by the (Rama miFano, Asarah Maamaros, Ma'amo Chaqor Din 3:21), saying that this position is not in dispute with the more usually cited 7, just giving more detail. Whereas Rav Aharon Lichtenstein ("The Seven Laws of Noah") only justifies his giving a longer list by stating that he is expounding on the standard 7.
An example of the breadth of each Noachide mitzvah. Abortion is prohibited to both Jews and non-Jews. However, it is not punishable when performed by a Jew. And for that matter, the Rambam distinguishes between cases where it would be permitted to abort a moment before birth, and prohibited to commit infanticide a moment after. (Hilkhos Rotzeiakh 1:9) In the opinion of many (most?) recent and contemporary decisors, abortion is not murder as the law is defined by Jews; it's another prohibition -- perhaps injury to the mother, or a more extreme form of the prohibition against masturbation (as both prevent a future person), yet more debate. But all agree that abortion is murder as defined in the Noachide Laws. "Murder" is an umbrella term that includes numerous prohibitions if counted the way the Jewish 613 were counted.