It has to be done in the script file.
If you look at the .bat file in your process directory, there are some lines that can be uncommented to get you started. Basically, it will create a date-based name and copy the file.
It looks like you are doing an insert/update. In that case, you could add a few lines to the script to copy the date-based file (i.e. 01032012-import.csv) to a static name that is set in process-conf.xml (i.e. import.csv).
Hope this helps,
Vijay