Hey,

Yeah man, of course they can. One can appear for the same through NIOS board. But what i will suggest is if you already have a PCB class 12th certificate why dont one just ads up a subject to it. It would be really beneficial for future purposes too and even if you score less on the other exam you can get away with it as it was a Extra subject that you opted for later.

Hope I solve your query.

All the best!!!!