Here's a start:
add network service tftp
add tftp client <ip address or hostname> this is the IP address of the machine to which you wish to authorize to connect to the NetServer using tftp.
Now, here's the part that I'm unclear on. The NetServer, as do Total Control ARCs, do not save their files as text. They're saved as some sort of binary file. So you have to define a bulk file.
What you want to, I think, is issue set BULK_FILE <filename>. Just call it like netserver.config, or whatever you'd like. Now when you do a save all it should save the config to NVRAM AND to the tftp server.
Worth a try. Like I said, it's been a LONG time since I've had to deal with a NetServer and I don't have much need to do this on the Total Controls that we currently have.