Yes, you can add Mathematics to your 12th marks by giving the NIOS exam and then become eligible for MHT CET.

You have passed 12th earlier with Physics, Chemistry, and Biology. Now if you want to apply for Engineering or BTech through MHT CET, you must also have Mathematics in your 12th. You can do this by applying through NIOS (National Institute of Open Schooling) and choosing Mathematics as an additional subject.

You don't have to give all subjects again. NIOS allows you to use TOC (Transfer of Credit). This means you can keep your old marks in Physics and Chemistry and only write the Mathematics exam now. NIOS will issue a new marksheet after you pass, including both the old and new subjects.

Once you clear the Maths paper, your subject group becomes PCM (Physics, Chemistry, Maths), which is valid for MHT CET Engineering admissions.

You need to apply on the NIOS website as a 12th student. While applying, upload your old marksheet and select TOC for Physics and Chemistry. Choose Mathematics as your new subject. After paying the fees, prepare for the exam. When you pass, NIOS will give you a fresh marksheet. Make sure to finish this before MHT CET admissions start.

website:
https://www.nios.ac.in