The Gemara (Megillah 2b) states that a prophet is not allowed to innovate a new law. If so, how can there be biblical laws whose source is a verse in a prophetic, post-Pentateuchal book? For example, some laws of mourning, which according to some opinions is of biblical force (at least on the day of burial), are learned from Ezekiel 24:17 (Mo'ed Katan 15a, 27b). Some laws of prayer are learned from Chanah's prayer in 1 Samuel 1 (Berakhos 31a). The requirement of separation of the genders is learned from Zechariah 12:12 (Sukkah 52a). How can these laws be biblical (i.e. from the Torah) if they are derived from a later prophetic book, and a prophet is not allowed to add to the Torah?
The Maharatz Chajes (Glosses to Nedarim 22b; Toras Ha-Nevi'im, Ma’amar Toras Nevi’im Divrei Kabbalah, chs. 3-4. Cf. The Students' Guide through the Talmud, ch. 5) quotes the Ramban in his glosses to Sefer Ha-Mitzvos (end of shoresh 2), who states that these laws were part of the oral tradition until the prophet wrote them down. The prophet did not add them, because they were in force since Sinai. Certainly, they should not lose force simply because a prophet wrote them down.
This is based on a passage in Sanhedrin 22b:
That which Rav Hisda taught [that an uncircumcised priest may not perform the sacrificial order in the Temple] we did not learn from the Torah of Moses, our teacher, until Yechezkel came and taught us, “No foreigner, uncircumcised in heart and uncircumcised in flesh, shall enter into My sanctuary to serve me” (Ezekiel 44:9). Until Yechezkel came who said it? It was taught as a tradition until Yechezkel came and supported it to the text.The Gemara states that the law prohibiting an uncircumcised priest from serving in the Temple was an oral tradition passed down from the time of Moses. When Yechezkel wrote it down in his book of prophecy he did not create this prohibition; he recorded it. The same can be said of the other obligations listed above. They were oral laws with full Pentateuchal force that prophets merely recorded.