Add a taxonomy term to many vocabularies
I don't think it's possible. It's always one to one relationship between vocabulary and term...

If you see taxonomy_term_data table which stores term id and vocabulary id reference below..



tid is primary key which means always unique. So, you can't have another row which points to different vid.

Below schema definition in taxonomy.install confirms same..

  $schema['taxonomy_term_data'] = array(
    'description' =&gt; 'Stores term information.',
    'fields' =&gt; array(
      'tid' =&gt; array(
        'type' =&gt; 'serial',
        'unsigned' =&gt; TRUE,
        'not null' =&gt; TRUE,
        'description' =&gt; 'Primary Key: Unique term ID.',
      ),
      'vid' =&gt; array(
        'type' =&gt; 'int',
        'unsigned' =&gt; TRUE,
        'not null' =&gt; TRUE,
        'default' =&gt; 0,
        'description' =&gt; 'The {taxonomy_vocabulary}.vid of the vocabulary to which the term is assigned.',
      ),
      'name' =&gt; array(
        'type' =&gt; 'varchar',
        'length' =&gt; 255,
        'not null' =&gt; TRUE,
        'default' =&gt; '',
        'description' =&gt; 'The term name.',
        'translatable' =&gt; TRUE,
      ),
      'description' =&gt; array(
        'type' =&gt; 'text',
        'not null' =&gt; FALSE,
        'size' =&gt; 'big',
        'description' =&gt; 'A description of the term.',
        'translatable' =&gt; TRUE,
      ),
      'format' =&gt; array(
        'type' =&gt; 'varchar',
        'length' =&gt; 255,
        'not null' =&gt; FALSE,
        'description' =&gt; 'The {filter_format}.format of the description.',
      ),
      'weight' =&gt; array(
        'type' =&gt; 'int',
        'not null' =&gt; TRUE,
        'default' =&gt; 0,
        'description' =&gt; 'The weight of this term in relation to other terms.',
      ),
    ),
    'primary key' =&gt; array('tid'),
    'foreign keys' =&gt; array(
      'vocabulary' =&gt; array(
        'table' =&gt; 'taxonomy_vocabulary',
        'columns' =&gt; array('vid' =&gt; 'vid'),
      ),
    ),
    'indexes' =&gt; array(
      'taxonomy_tree' =&gt; array('vid', 'weight', 'name'),
      'vid_name' =&gt; array('vid', 'name'),
      'name' =&gt; array('name'),
    ),
  );