We are using ECXpert 3.6.3.
Is it possible to "upgrade" or replace the ECXpert FTP server with an ftp server that is more robust?
If not, is it possible to modify the existing ftp server by using plug-ins? If so, where can we get information/documentation on doing this?
The ECXpert FTP server is a custom ECXpert server that supports the 'submit' function (as well as a couple of other ECXpert specific functions). It is integrated as an ECXpert server. It can't be replaced within ECX but it can be replaced ("kind of...") by any ftp server: your remote client can use any FTP server, instead of the ECX server to deposit the files where a perl or shell script could 'submit' them. Or you could use the ECX FTP client to pick them up and submit them.
I seem to recall that ECX also comes with a slick automated FTP client that allows scheduling and supports SFTP.
In summary, if the ECX-packaged FTP server doesn't address all of your requirements, there are a multitude of options for submitting files that should help.