I am afraid you can not create folders in Alfresco from PRPC using CMIS. From PRPC we can put files in any pre created folders. But if alfresco allows any java API or service [soap/rest] for the similar functionality then you can create connectors and try the same.
Please let me know if there is any follow up question on this.
Dont think you can not change the directory setting on the fly and based on your logic using CMIS. Because in CMIS we select folder to store the document as part of CMIS connector and it is not possible to change it.
Sorry for my ignorance if my suggestion does not serve your purpose, but based on my googling I could see there are REST API available to create folders and also upload files in those folders . These API allows you to pass name of directory to create and to upload files as well. Please check few links for more information.