Yes setting permission in a workflow is very possible. Assuming that folder created by the same user that will assign those permissions. You can try the following in your script (which can be found on the Alfresco Docs site):
folder.setPermission("username", authority);
authority means the type of permission you want for that user, which can be either of the following: 1. SiteManager or Manager 2. SiteCollaborator or Collaborator 3. SiteContributor or Contributor 4. SiteConsumer or Consumer 5. Editor.
