You cannot append XML logs. If would create invalid XML.
You can create different log for every run with !T pattern:
https://winscp.net/eng/docs/ui_pref_logging

Hi I'm using scripting of WINSCP with XML logging. The problem is that the file is overwritten each time it logs.
In the GUI I can see a button "append to file", is this option available with scripting?

Using version: 4.2.9

Command call:
"C:\Program Files (x86)Winscp\WinSCP.com" /script="D:\ftpscripts\ftp_getfilesWithMove.bat" /parameter /fromFolder/ D:\ToFolder\ *.xml /xmllog=D:\Log\Log.xml

The Script looks like:
option batch abort
option confirm off
open ftp://user:password@server:port -passive
cd %1%
lcd %2%
lpwd
get %3%
mv %3% ./Archive/
exit

Regards,
Rickard