SFTP upload - Ignoring bad data

I'm currently trying to set up a scheduled SFTP data upload of a csv file from an older legacy system which is rather inflexible with it's data structure. Most of the issues I've found solutions to, but I've run into a particular issue with a custom object that I can't solve.

The file contains a number of date fields.. but unfortunately when this system has a null value for a date, it enters "--/--/----" into the cell. On import, Eloqua takes one look at this, declares it mismatched data errors and throws out the entire row of data.