Yes, you can get admission in pmsss if you already studying in any different college, but make sure that you have not benefited from pmsss in past. Also to apply for pmsss 21 the primary condition is that you should have passed your 12th in 2018-2019 or 2020-2021 academic year i.e this year or the previous year . You should have done your 12th from J&K only and should have a Domicile of UT of Jammu and Kashmir or Ladakh .
You can apply to pmsss if you fulfill all the above requirements. make your you don't cancel the admission of your previous college unless you are not alloted a new college under pmsss, and later when a college will be alloted to you through pmsss you can leave the previous college.