If you want to add Mathematics as a subject in your 10+2, then you can obviously appear for the NIOS ONDEMAND examination and you can qualify only the Mathematics. As NIOS is a nationally recognized open school its score are valid for most of the examination. So you can do so and also you can appear for the PCB group as well as PCM group in MHTCET if you qualify the NIOS On demand examination.

For more information regarding NIOS consider the link below:

https://school.careers360.com/exams/nios-class-12

For more information regarding MHTCET consider the link below:

https://engineering.careers360.com/articles/mht-cet-exam-dates

I hope this helps. Wish you a great eve!