@sagrawal - As per my understanding there is no such importer which just imports the external site by providing the endpoint. Your external side should either expose the data using XML or JSON which can be consumed in AEM to create pages and site. Same way you need to get the styling as part of clientlibs. So migrating one or two pages is easy but whole site will require a kind of whole new setup.
@sagrawal Could you please elaborate your requirement ? It will depend on what you want to do? Are you planning to migrate an existing website to an AEM website ? Or is this something else. I was involved in a migration in which the website content was provided to us in XML formats in more than 300 zip files. And migrating that to AEM , is not easy task or something which is OOTB , you have to design the migration script according to the existing content and the components you have developed in AEM . A simple answer would be really difficult for this question.