There is a misconception here, and I’m not sure at what level. I hope the following will address the issue.
We know that after Moshe Rabbeinu even Neviim weren’t able to add to the Mitzvos. When the Yidden forgot 3,700 Halachos after Moshe Rabbeinu was Niftar, Hashem told Yehoshua that He can’t fill him in.
However, the Halacha of Lo Sassur still applied. The Neviim were the ones who passed down the Torah and when they interpreted a Halacha we listed to them as a Talmid to a Rebbe; when they made a Gezeira we listened because of Lo Sassur; and when they said other things it was a Mitzva to listen to a Navi.
After the Neviim, when we had the Anshei Knesses Hagedola and the Tannaim that followed, they still didn’t have permission to add new Mitzvos. But, they continued to have the right to make Gezeiros and Takkanos.
The Gemara tells us that Shlomo Hamelech made other Gezeiros as well. He added another generation to the list of Arayos. These are Derabanan. They aren’t from Nevua. Ezra created Muktza. According to some, certain aspects of Muktza is a Derabanan from Moshe Rabbeinu. Ezra is also probably the one who Assured Kinyanim on Shabbos.
We use the term Chazal (Chachamim Zichronam Livracha) to refer to the Chachamim that show up in the Mishna, Gemara and Midrashim. However, the Torah, Halacha, Gezeiros, Takkanos, and Talmud, didn’t start after the Anshei Kneses Hagedola. Shlomo Hamelech taught Torah and gave 3,000 reasons for every Mitzva De’oraysa and 1,005 reasons for every Derabanan (Eiruvin 21b). This shows us that he was a teacher, that there were Derabanans, and that Derabanans are very deep and have many Taamim.