I think, this can be achieved using following steps (not sure whether there is any OOTB solution for this or not).
1) Create a Java program, which can read certain directory for any new files and create .jar.
2) Import .jar in to IBM BPM and create a Java integration service.
3) Create timer based UCA (which fires on specific frequency of time) , attach above service with the UCA.
4) Write additional logic to start the process on arrival of any new document (using tw.system.startProcessByName)