Hello Subhajit,
To my mind, at least with Adobe Campaign standard version (I am not sure for ACS), it is possible to use the factory Adobe Campaign mechanism for web tracking, only if your "external" web site is actually either an Adobe Campaign standard web site (called AC webApp) or an Adobe Experience Manager (AEM) web site.
Otherwise you need any tagging tool such as Adobe Analytics (Omniture), Google Analytics, AT Internet/xiti, etc, or even better Adobe DTM.
Regards
J-Serge