A directory can only have one owner, but you can set up some sharing:
In my case, I create a new FTP user every time I create a domain so that all domains are separate and it's easy to just hand it off to someone else by giving them the FTP login info. You can do this right now with an existing domain.
Go to the panel here and then Domains -> Manage Domains then click Edit for that domain. You can then change the FTP user, but it looks like you'll have to copy the data over. I could have sworn that there was an option to have this done automatically. Maybe someone else can jump in with more info on this.