Yes, just add it to the root users' crontab; run the crontab -e command.
crontab -e
The places cron stores its files can be a little bizzare, so use the crontab -e command which will make sure it's in the right place, and I believe it checks the syntax.
You can just do
as root.
Just specify root as the user for the entry in /etc/crontab:
root
/etc/crontab
0 0 * * * root somecommand
Alternatively you can also add the command to root's personal crontab by using crontab -e as root.