You can write information to an external file, but after the execution of your program, all the files are wiped out. The files you create in the editor are sent to the jail server to be part of the execution environment but never are retrieved.
At this moment VPL can't retrieve files from an execution in a jail server. The nearest feature to what do you want is this: After running your program with the evaluation option, the output (standard output and standard error) of the evaluation program is retrieve as the evaluation result. The last evaluation result is saved with the submitted files.
Also, you can write an evaluation program to process the file content and generate an appropriate output.