As with the supernatural generally, humanists are also not very concerned with gods specifically. Indeed, if it weren't for so many believers insisting upon the importance of their various gods, humanists probably wouldn't have much at all to say about their possible existence. The Humanist Manifesto of 1933 doesn't even mention gods, the second Humanist Manifesto of 1973 mentions God just three times, the Humanist Manifesto 2000 once again has no mention of any gods, and the Humanist Manifesto III does not mention gods but only states that humanism is a philosophy "without supernaturalism."
Thus, gods are essentially a non-issue for the basic philosophy of humanism itself - they are only made significant by the absence of any focus upon them. Because of this, humanism doesn't necessarily exclude a belief in the existence of gods. A humanist who is also a theist may not be especially common, but it also isn't self-contradictory and so is quite possible. Even a philosophy of secular humanism is not incompatible with theism - secular humanism is secular because it is necessarily non-religious, not because it is necessarily non-theistic.
For a humanist (especially a secular humanist) to also be a theist, what must happen is that the theist must not use the god(s) in question as a way to explain the existence of the universe, the existence of humanity, the existence of morality, etc. In essence, their humanist beliefs must be independent of their theistic beliefs, with the former not depending upon the latter. As the Second Humanist Manifesto states:
"We believe... that traditional dogmatic or authoritarian religions that place revelation, God, ritual, or creed above human needs and experience do a disservice to the human species."
"[W]e can discover no divine purpose or providence for the human species. While there is much that we do not know, humans are responsible for what we are or will become. No deity will save us; we must save ourselves."
Both of those statements express very important and fundamental principles of humanism, but neither of them is incompatible with theism. The Second Humanist Manifesto does suggest that humanist must be nontheists; insofar as theism is restricted to the traditional, philosophical theism derived from Judaism, Christianity, and Western philosophy, that is probably true. It is unlikely that a person would be a humanist and believe in the existence of a God with the attributes mentioned above. But, because that isn't the only sort of theism which exists, the possibility of a combination of humanism and theism nevertheless exists and may increase over time as the diversity of theism in the West also increases.
That development may be one reason why the issue of gods and theism simply wasn't mentioned in the Humanist Manifesto 2000. As indicated above, theism simply isn't relevant to humanism - neither the presence nor the absence of theism plays any important role in the positive philosophy that humanism offers. So long as a person's other beliefs and other philosophies of life, whether theistic or atheistic, don't explicitly contradict any fundamental principles of humanism, they are compatible with humanism.
Does this suggest that humanism doesn't really mean anything if a person can be a theist and a secular humanist? I don't believe so. It's important to understand that secular humanism is not a dogma. Nor is it a doctrine, a creed, or a set of rules that a person must sign off on in order to become a "member" of a club. There is no humanist or secular humanist pope who can set down infallible doctrines that every humanist must believe.
No, secular humanism is a set of principles, perspectives, and ideas about the world. Secular humanists are allowed to disagree - not only on the conclusions they draw from those principles, but even on the trust, formulation, and extent of those principles themselves. Just because a person doesn't happen to subscribe 100% to every phrase and statement that appears in humanist documents doesn't mean that they cannot be secular humanists. If that were necessary, then that would make secular humanism meaningless and there wouldn't be any real secular humanists.