Hi Anuj,
you could create
copy of CMSSiteMap web part and put dropdown list directly into this copied web part.
Then you can set WhereCondition property of SiteMap dynamically in code-behind of the web part in according to value currently selected in dropdown list:
this.smElem.WhereCondition = "DocumentID IN (SELECT DocumentID FROM CMS_DocumentCategory WHERE CategoryID = " + drpList.SelectedValue.ToString() + ")";
Best Regards,
Martin Dobsicek