You need to start with fissile material to make weapons grade fissile material. If there is no peaceful application for fissile material then a global ban treaty complete with trust-but-verify policies is within the realm of reason.
Regardless of this hypothetical: the fact remains that no part of a fusion reactor increases weapons proliferation risks, unlike a fission reactor. You could plop one down anywhere on the planet and locally source fuel.
If you want a neutron source then you can make one in your basement with a fusor or linear magnetic mirror.
Nuclear fusion emits neutrons.
If you can soak up surplus neutrons in uranium 238 you can breed plutonium 239.
By design intent fusion does not intend making fission happen. A side effect of the nuclear physics makes fission products, if you want to.
You can make weapons grade fission materials, with neutrons from a fusion reactor.