There are at least two ways to do it, both with important limitations:
1) Allow root ftp access.
This not recommended and is extremely dangerous, as it will pass the root password in cleartext. Think carefully before you do this; it is only a matter of time before your system his compromised; maybe even a short time.
2) Do the same as above, but allow only scp access, not ftp access. Some ftp clients manage scp connections as well.
If I were to do it, which I'm not, I wouldn't use the actual root login but would create another login with root privileges. Remember tha anyone with this username/password can completely destroy your system, either intentionally or accidentally.
Jeff