Well, easily, the CPanel system will allow you to do this:
E.g. you created a subdomain called "sub1"
Now if you create a FTP account with the username "sub1" and try logging into FTP, you'll find that the account is locked into the sub1 subdomain folder only for file transfers - this is done automatically by CPanel.
I'm not sure how you can add additional FTP users for the same subdomain as such above using another username for FTP users.