I tried that. I washed out after one year, because I couldn't sustain the laser-focused interest on the single area I was supposed to be studying in grad school. At an entire large state university of tens of thousands of students and classes on every subject, I was only supposed to take ones the advisors deemed "in my field".
There might be universities out there with "a la carte" grad programs, but I'm not aware of any.
There might be universities out there with "a la carte" grad programs, but I'm not aware of any.