Yes, this is possible. Simply go to your dashboard and to "All participants". Find the participants you would like to reward and click on "Complete submission" for each of them. This participant will now show up in "Participants ready for review" and can be reviewed just like all the other participants.
If you are using the new site https://beta.prolific.ac/ you can view all submissions for a study together, but going to "View Submisisons" You can then reward the participants by selecting the green tick on the right next to the participant's submission.