Hi,

Am trying to build a search platform with SharePoint 2013 Enterprise where I need to implement a different search design.

I will configure Enterprise SharePoint Search and create indexes and crawl my desired content sources. In addition to that I need to import search index files from external applications as well and combine both the Enterprise SharePoint Search index and these external index. Am clueless as to how I should be doing? Is it even possible in SharePoint?

so SP doesn't "import search index files"... it indexes external applications itself... directly... using content sources.

Otherwise though, using SharePoint as an enterprise search engine/portal is a great use.

The only way to connect to the external applications are through their APIs and these APIs send their search index when we invoke them, they don't want us to crawl their content and create our own index.
How do I handle this scenario?

Can SharePoint consume this search index?

custom connectors

see: http://www.sbrickey.com/Tech/Blog/Post/SharePoint_BCS_-_Sample_Net_Assembly_Connector

slides: http://www.slideshare.net/Scott_Brickey/sharepoint-saturday-dayton-2012

thought i had the code in codeplex as well... looks like it's just in the blog post... either way, completely possible.