The IP address is used to find the server. It's not part of the ftp protocol.
Once the domain resolves to the proper IP address, it can be used to find the server, but otherwise, it's not part of the ftp protocol either.
So that shouldn't matter.
The site user logs in as:
server: ip.nu.mb.er
login: username
password: password
And that should work the same as:
server: example.com
login: username
password: password
Jeff