How are you planning to use this mongo db data? Is it just to read final processing results?
To read the data and convert to an ExampleSet will be suitable for you? Nevertheless, you can read the data from mongodb to csv, using this application: http://docs.mongodb.org/manual/reference/program/mongoexport/
After to be converted to CSV, Rapidminer is able to deal with this. You can automate everything with a run script and read csv operators.