SmarterTools seem a bit disinterested in making this happen, I suggested it here maybe 5 years ago. Opened a ticket about it last year and was told it was being worked on, coming soon etc, but a dozen releases later there is still no sign of it :(
In my opinion you'd be out of your mind to use plain text ftp service on a production server these days, it has to be ftps for PCI compliance and all around good security, but then SS can't fetch the log files for analysis. This conspires to make SS, which we have used since v2, all but useless for our purposes. Our old unlimited site license has been steadily downgraded to a 50 site license as ftps has become the norm, and our ability to actually use SS has diminished. Pretty soon we'll have to ditch it altogether if the developers don't wake up.