Can I use FTP for project file sharing/transfer?

I work with a few other individuals on various online projects involving frequent transfers of video and audio material, often around 1GB at a time, and requiring up to about 10-20GB of content available online at any given time. Until now we've used the iDisk feature in Apple's MobileMe, but as MobileMe is being shut down we have to look at alternatives. Since I already have an InMotion hosting account I thought this might be a simple, free way to continue working in the same way - I found Panic Software's Transmit, seems like a great app to simply manage the transfers as none of us are very technical.

I guess my question relates to the actual FTP address – how can I have the FTP address reflect one of my Addon Domains rather than my Main Domain? For example, let's say my main domain is "site_a.com", and the domain associated with these projects is one of my Addon Domains, "site_b.com" – how can I set up the FTP site so the address is ftp://ftp.site_b.com/ rather than ftp://ftp.site_a.com/?

Any advice appreciated!