there are two cases of additional biology

1) students who have study pcmb in the same year and have marksheet of the same year

2) student who has study pcm in let's said the year 2016 and then give biology exam next year as a single subject

if you fall in case one then no problem go ahead 

if you fall in case 2 then you are eligible to fill the form for neet but the candidature is subject to an outcome on 16th April 2018