If you don't break the same-origin policy, you can use:
window.parent.document.getElementById()
Sample
h1.html
<select name="selectYear" id="selectYear">
<option value="2000">2000</option>
<option value="2001">2001</option>
<option value="2002">2002</option>
<option value="2003">2003</option>
<option value="2004">2004</option>
</select>
<iframe src="h2.html">
h2.html
<script type="text/javascript">
function addYear(year){
var select = window.parent.document.getElementById("selectYear");
select.options[select.options.length] = new Option(year, year);
}
</script>
<button onclick='addYear(2005);'>Add year 2005</button>