The short answer is to do a partial reload if you want to keep the data that was previously loaded and add new data with another reload. Search for partial reload in the help.
However it is hard to always depend on partial reloads. A better approach would be to save the csv files you are loading as QVD files in the script with store function (or just one QVD file) and then load that QVD file with the previous data, concatenate the new CSV file to that, and save the complete data over the old QVD file everytime you do a reload.
I don't know why you are loading one CSV at a time, but you can also load multiple table files that have the same structure using wildcard symbols to refer to the file name. (e.g. Sales*.csv)
