Your question is really more of an ftp question. The same thing could be asked when using any other ftp client. The real question is whether the ftp protocol allows you to modify a file on a server without first downloading it, changing it, and then uploading the new file. The ftp protocol has an APPE command which allows you to append data to an existing file on the server. If the file doesn't exist then the server is supposed to create it. As far as I know the Internet Toolkit does not have a VI to implement the APPE command. So if you wanted to do this you would need to do this yourself by probably creating your own ftp VIs. OpenG had an ftp library that you could probably use. You can get it here: http://wiki.openg.org/OpenG_Internet_Connectivity_Tools
I can't seem to find anything about the OpenG Internet Connectivity (OGIC) Tools. The link provided in your post is broken. The OGIC SourceForge page (http://sourceforge.net/projects/ogic/) seems to be dead / incomplete. This code doesn't seem to be in the current OpenG library. Any idea why? Or where I can find the OGIC Tools?