You could use this method to custom your drop-down list top link bar in SharePoint 2010,Easy and Simple SharePoint 2010 Dropdown Navigation bar that works no scripts, no web.config, works on hosted providers.
Here is my code in a custom Master.page I used SharePoint Designer to set the DataSourceID=SiteMapDataSource1 – in the GUI click the TopMenuNav bar click the top-right > arrow and then “New Data Source” use the default name.
Code should look like this. I also changed the SPSiteMapProvider under
SiteMapProvider in both ID= under AspMenu and SiteMapProvider under
Then you will find your sharepoint site’s top link bar like this:
I think Microsoft they should add this feature by default.