To "overwrite" a table you can use PHPMyAdmin to "Empty" the table. If your SQL dump creates a table, you can also "Drop" the table after you "Empty" it. You can't add or "Drop" databases using PHPMyAdmin, that has to be done using the DreamHost Web Panel.
The best way to import an SQL dump is through the command line in a shell, because it is more direct with less caveats:
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