We have an enhancement slated for a future release that will let you use a Channel Cache or System Variable in the host field for an FTP/SFTP source. Using this method, you can access that information from within a mapping node by using channelCache.getValue() or qie.getVariable(). Look for enhancement 4448 in the release notes of future releases.
Until that feature is relased, though, there is unfortunately not a way to programatically retrieve the host from the source node.