Extend a modules database table
I'm a litle confused about your goal, but taking your question literally as I would like to extend the schema of another module, and assuming by extend you mean alter and not inherit from, then yes, it's pretty straightforward.

First of all, implement hook_schema_alter() so that Drupal knows about the change you're going to make to the table:

function MYMODULE_schema_alter(&amp;$schema) {
  $schema['rooms']['foo_table']['new_column'] = array(
    // ...
  );
}


Then add an update hook to your module's install file to physically add the field to the table, e.g.

function MYMODULE_update_7100() {
  $schema = drupal_get_schema_unprocessed('rooms', 'foo_table');
  db_add_field('foo_table', 'new_column', $schema['new_column']);
}


It would also be advisable to implement hook_uninstall()/hook_disable() to remove that column from the table when your module is no longer active. Leaving it there probably won't hurt anything, but for the sake of cleanliness it's a good idea.