The gemara learns out the concept of כל המוסיף גורע from chava who told the snake that she wasn't allowed to even touch the tree and as a result misled her into eating from the eitz hada'as by pushing her into the tree and showing her that there is no consequence from touching the tree. In my sefer, Nasiach B'Chukecha (page 267) I quoted the Minchas Chinuch who proves from this gemara the opinion of the Rambam against the Ra'avad. The Rambam and Ra'avad argue about when the the Rabbonon institute a gezeira, whether it is necessary for them to make it clear that the nature of the prohibition is only d'rabonon and not d'oraysa. The Rambam insists that the Rabbonon be clear that gezieros such as not eating chicken and milk together, are only Rabbinic in origin, but by claiming it has d'oraysa sources they would be in violation of ba'al tosif. The M.C. explains that if adam sinned by telling chava not to touch the tree, it makes sense that we derive from the episode a concept of כל המוסיף גורע, but if we consider the safeguard implemented by Adam to be a positive contribution, why does the gemara consider it an aveira? Nonetheless, even within the opinion of the Rambam it isn't clear that there would be a violation for NOT saying that it is d'rabonon. The violation of ba'al tosif may only be when you claim explicitly that it is d'oraysa. Presumably Adam didn't claim that Hashem told him not to touch it, rather just told chava not to touch it, so even according to the Rambam it is difficult why we consider this to be a violation.