You get to set the permissions a MySQL user has using the webpanel. To do so, click on the "Modify" button next to the MySQL user for the database in question.
The permissions are: Select, Insert, Update, Delete, Create, Drop, Index, Alter.
The privileges are described at Reference Manual - 5.6.3. Privileges Provided by MySQL
Once these permissions are setup appropiately, the MySQL user can use PHPMyAdmin to add or remove tables or through any other mysql client.
Hope this answers your question. I don't know what you mean by a "subdomain user", the types of users are found here: http://wiki.dreamhost.com/index.php/Users
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