Copying should work just fine. What I'd suggest is editing the file and not moving it or copying it. That way you really shouldn't have to deal with permissions, especially if you're new to Linux.
While you have an SSH session going from another machine, with a copy of wpa_supplicant.conf open on the machine your SSH'ing from, do the following:
sudo touch /etc/wpa_supplicant.conf
sudo vi /etc/wpa_supplicant.conf
Copy the text from the wpa_supplicant.conf file you have open on your host machine. Then on the ATV perform the following while vi is open.
1) Type in "i" for insert in the vi session
2) Paste in your text, the right mouse button should work for that
3) Make sure all of the text is correct
4) Hit the escape (esc) key
5) Type a colon (:)
6) Type wq (write then quit) then enter to save the file.
7) Check your file with "cat /etc/wpa_supplicant.conf"