Can you add a map (either AMMAP or GoogleMap) on an existing Dashboard

Yes, this is possible with both mapping solutions although the way you implement it is quite different. I can give you an example on how to embed the Google Maps module in your custom dashboard. This is a snippet for the advanced XML, you could use (the Google Maps app has to be installed):

<module name="HiddenSearch" group="Map View" autoRun="True" layout_panel="panel_row1_col1">
    <param name="search">sourcetype=access_combined NOT clientip=10.* NOT clientip=192.168.* | geoip clientip | genormalize | stats count as _geo_count by _lat,_lng</param>
    <module name="GoogleMaps">
        <param name="autoPostProcess">false</param>
        <param name="height">350</param>
        <param name="mapType">terrain</param>
        <param name="mapTypeControl">on</param>
        <param name="navigationControl">on</param>
        <param name="scaleControl">on</param>
        <param name="scrollwheel">off</param>
    </module>
</module>

There's some info on how to embed AMMAP in your custom dashboard in the Splunk Documentation: http://www.splunk.com/base/Documentation/latest/Developer/CreateMap