I have NAS units that run windows server and would love to backup to them w/o having to install a FTP service.
This one is easy. Just mount them and treat them like a regular directory (there’s no other way to talk to NFS). 
All of the backup types in Webmin and Virtualmin can be configured to work with a regular file or directory.
And, of course, you can mount NFS filesystems using the Webmin Disk and Network Filesystems module.
Or, if you only wanted it mounted while backing up, you could add a command like:
mount -t nfs 192.168.1.1:/path/to/directory /mnt/backup
In the "Command to run before…" field. Where 192.168.1.1 is the NFS server, /path/to/directory is the location on the file server, and /mnt/backup is a mount point on the Virtualmin server.
You may find that SMB/CIFS works better, though, since you’re talking about a Windows server. Last time I used NFS on Windows (admittedly years ago) if was sadly inadequate, but SMB/CIFS worked pretty reliably. Linux works fine with either.
ok, easy enough 