Migrating multiple values into a single file field in Drupal 7 with Migrate 2.2
I think what you need is prepareRow. Here is code from the migrate baseball example where multiple fields are merged into a single array field.

 function prepareRow($row) {
    // Collect all the batters into one multi-value field.
    for ($i=1; $i &lt;= 9; $i++ ) {
      $key = "visiting_batter_$i";
      $visiting_batters[] = $row-&gt;$key;
      $key = "home_batter_$i";
      $home_batters[] = $row-&gt;$key;
    }
    $row-&gt;visiting_batters = implode(',', $visiting_batters);
    $row-&gt;home_batters = implode(',', $home_batters);
    $row-&gt;title = "$row-&gt;home_team vs. $row-&gt;visiting_team. " . gmdate('M d, Y', strtotime($row-&gt;start_date));
  }


(from the file migrate_example_baseball.migrate.inc)

However I have been having mixed results with prepareRow in my own code, so while this looks straight forward I can't guarantee that it will work.