I do not think so but if you did conferences or won awards, you can put that under like experience ( I cannot remember all the categories) and just explain your awards and such. That is what I did.
I asked my early decision school about this and they said that PTCAS does not allow for students to upload research posters by themselves but if one of your references is a professor you worked with on those research projects then they can possibly mention it. Students are only allowed to mention research work under the experiences category but you can explain it or if there is a link to a website where your research posters are listed you can include that in the description.