Site to mobile app

Hi,
We have a site written in built using ASP.NET MVC 4, ASP.NET 4.5 with a searching algorithm. We want to create a mobile app for android without having to write everything from scratch. Should we add and an API for the site? Can the same developer writing the mobile app do it or do we need to find two different developers?
Thanks,

You can use the same developer if that have the coding experience in ASP and the mobile app code.

You can implement your app as hybrid so that you can retain your CSS and parts of HTML. And you can then use existing AJAX apis for search.
To do that you need so called hybrid client, the most popular one is Cordova.