you can change system variables by using assign block after the entry, or you can right click on the callflow and edit it with text editor instead of callflow editor and edit the value of the variable,
regarding passing parameter to subdialog, you open the subflow and edit the entry variables to be input variables instead of user, then get back to the subdialog block press on the parameter property and you will find it as input,
if you want to make it output, go the the subflow exit block and add the parameter to the output