Implementing your own source class - say you're migrating from a database system not supported by Migrate, or from some unusual file format - is not that difficult. You simply need to derive a class from MigrateSource, and implement a few simple methods (well, how simple they are depends on your source...).

Here's a very simple source class that generates ten sample source rows. For more sophisticated examples, review the source classes implemented by Migrate itself in plugins/sources.

class SimpleMigrateSource extends MigrateSource {
  protected $currentId;
  protected $numRows;

  // Your constructor will initialize any parameters to your migration. It's
  // important to pass through the options, so general options such as
  // cache_counts will work.
  public function __construct($num_rows = 10, array $options = array()) {
    parent::__construct($options);
    $this->numRows = $num_rows;
  }

  /**
   * Return a string representing the source, for display in the UI.
   */
  public function __toString() {
    return t('Generate %num sample rows', array('%num' => $this->numRows));
  }

  /**
   * Returns a list of fields available to be mapped from the source,
   * keyed by field name.
   */
  public function fields() {
    return array(
  		'id' => t('ID'),
  		'title' => t('Title'),
  		'body' => t('Body'),
    );
  }
  
  /**
   * Return the number of available source records.
   */
  public function computeCount() {
    return $this->numRows;
  }

  /**
   * Do whatever needs to be done to start a fresh traversal of the source data.
   *
   * This is always called at the start of an import, so tasks such as opening 
   * file handles, running queries, and so on should be performed here.
   */
  public function performRewind() {
    $this->currentId = 1;
  }

  /**
   * Fetch the next row of data, returning it as an object. Return FALSE
   * when there is no more data available.
   */
  public function getNextRow() {
    if ($this->currentId <= $this->numRows) {
      $row = new stdClass;
      $row->id = $this->currentId;
      $row->title = 'Sample title ' . $row->id;
      $row->body = 'Sample body';
      $this->currentId++;
      return $row;
    }
    else {
      return NULL;
    }
  }
}