Can I set any username I like to register in the TeraStation?

A username/password you register in the TeraStation as a local user should be the same typed to login to the Windows.

Otherwise, you may not able to access shared folders to which access is limited properly.
In addition, the access limits configured on shared folders may not function properly.

Example:
For Windows XP, register the same username/password as typed to log into the Windows in the TeraStation.

When the TeraStation’s access limits are operating normally, if accessing a shared folder configured as access-permitted, you are allowed to access to the folder without entering a username or a password.
When accessing a shared folder configured as access-prohibited, a message to ask for entering your username/password is displayed.