Besides the Safeguard configuration there is another possibility if you are using native Guardian users, this does not work for aliases.
If you want the native user to get a Guardian prompt you can change the portconf file of the related LISTNER process:
ftp $system.ztcpip.ftpserv UEXT 2000,2000,200 GUARDIAN 1
You need to restart the LISTNER afterwards.
The UEXT parameter increases the default file size and GUARDIAN 1 causes an implicite "Quote Guardian".
If you are within OSS you can transfer to Guardian subvols as well:
put abc /G/system/work/abc. This works fine for binary transfers, for ASCII you would get a code 180 file which could be converted to a 101 file by using CTOEDIT.