I have a domain for my host of the type http://a.10host.com. On the FTP server, the domains folder contains a subfolder a.10host.com. Can I create another subfolder named b.x10host.com to define a new domain?
No, just creating the folder will not create a domain, it will merely create a folder extra.
You would need to add it through DirectAdmins interface, under domain setup (there's a button called "Add New"). This also verifies that the domain is not already in use with any other account.