China has been building and launching it's Long March serious of space rockets and is making more and more launches, and with it's continued brisk SRBM and possibly IRBM production, it has probably the most up-to-date and readily available human resources and industrial base of any country in the world to start mass producing ICBMs.
Obviously SRBMs and satellite carriers are not ICBMs, but the skills needed to make the former could easily be transferred into the mass production of the latter. It will take a bit of time and a lot of effort, but it will be much quicker and easier than having to set up all the infrastructure and train up the people from scratch.
Even then it will not come close to 100 DF31s per year, but who needs 100 DF31s to be produced each year? Last I checked, we only have one earth to blow up, and even 50 DF31s will put a big enough dint in this earth for us all to be royaly screwed.