Hello there ,

Hope you are doing well . Yes, you can definitely just study Biology as an additional subject from NIOS. Students who have studied from NIOS are eligible for NEET. You need to score an aggregate of 50% in the three subjects i.e. Physics, Chemistry and Biology in  12th standard to be eligible for NEET,given that you belong from general category. For OBC,SC and ST ,the percentage is 40% and for PWD it's 45%.

Hope my answer helped.