If it really, really, REALLY wanted to, China could probably restart fissile material production of Pu without anyone noticing. It would involve a lot of money, a lot of equipment, and a lot of time, but it could be done. Production of HEU would be a lot easier to hide, but China's fissile material is Pu, so that isn't too relevant. This raises another question though: why in the world would China want to? China currently retains around 300 warheads, and this is more then enough. The absurd stockpiles maintained by the United States and the Soviet Union are relics of an paranoid and irrational Cold War mindset and have absolutely no basis in reality. Cold War estimates gave forty 1 megaton warheads the ability to instantly kill 20 percent of the U.S. population, even disregarding the later deaths from factors such as radiation and destroyed infrastructure. Given that the percentage of the U.S. population that has moved into cities since then, the human toll from the same amount of warheads is likely to be far more significant today. Unless China has designs to move away from its minimum deterrence nuclear policy, which it has followed faithfully for decades, and into some sort of counterforce posture, China's current nuclear stockpile is more then enough.
What China currently arguably lacks is effective delivery systems. The introduction of the DF-41 does improve the survivability of the Chinese nuclear forces by quite a bit, but last I checked they have not been widely deployed. DF-31 and their variants are mobile and thus survivable, but they have limited range. DF-5s, which make up the bulk of China's silo based ICBMs, are very vulnerable and are liquid-fuelled, meaning they must go through a lengthy fueling process before they can be launched. As far as BMD, current BMD systems are very limited, and have never (to my knowledge) been tested against countermeasures, which Chinese missiles are sure to have. Limited warheads are not an issue here if countermeasures are effective, and it shouldn't be possible to discriminate the re-entry vehicle with the warhead, which would mean that all vehicles would need to be intercepted. If China can demonstrate that its nuclear weapons are survivable and have a good chance of reaching their targets, deterrence is assured unless the U.S. elects a raving suicidal lunatic to the White House. The number of warheads China has, in this case, is enough, more then enough, and even borders on excessive.