AFAIK, there's no such functionality in WinINet API.
As workaround (if FTP file isn't big), you can download file from FTP, append your file to the end of it and upload again. If it doesn't work for you, consider using Ed Rauh's API_APPRUN in .VCX form File #9477 to run batch file.

>I ‘m using the VFP 5. + FTP Service Class
>And I need a little help cause I have limited experience with DLL
>I need to append to a file on the FTP server
>I was doing this with a .bat file and the RUN mybat.bat command
>
>

>@echo off
>c:
>cd \ASN
>ftp
>open 999.999.999.999
>myuser
>mypasswo
>cd ocum
>append test test
>bye
>

>
>This approach work but there is the DOS window opening and closing in run time
>To avoid this I like to use the FTP service class but it has no function for appending a file.
>I don’t know all the functions available on WinINet.DLL
>I understand that some of the syntax goes like
>

>DECLARE Integer InternetGetLastResponseInfo IN WinInet ;
>	Integer @nError, String @lpcBuffer, Integer @nMax_Path
>

>
>But how do I declare an append function?