Indeed, DSS build its own PYSPARK_SUBMIT_ARGS. Currently, there is no way to directly manipulate the spark-submit command line. All options of spark-submit can also be set by configuration properties (spark.driver*) ... except --packages
At the moment, you won't be able to use the --packages option. However, note that this option is a shortcut to retrieve+cache jars from Maven. You could retrieve the jars manually by writing a small ivy.xml, and then add the jars to spark.driver.extraClassPath
A note though: the DSS Spark API already includes the ability to read DSS datasets, including CSV ones, without the need for any additional packages.