Is it impossible to GPU mine without OpenCL?
OpenCL allows all graphic cards to be handled in a higher level in a uniform fashion. As all GPU mining software relies on OpenCL, you would need to create your own software to mine on non-OpenCL card. Whereas it is possible, the process would be very complicated and the reward probably wouldn't be worth it (NVIDIA cards aren't good for mining).

So all in all, no, if the graphic card does not support OpenCL, you can't use it to mine.