I found an answer in following link: How to Configure FAST Search in Sharepoint 2010
As in the answer, we can configure the Fast Search Server 2010 to crawl content a web sites in SharePoint 2007. Could someone please help share knowledge?
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You simply need to setup a new SharePoint content source and put the URLs to the sites to crawl from SharePoint 2007.
You may need to set your search service account to have Full Read access to the SharePoint 2007 web applications in Central Administration on 2007. To do that, goto Central Admin on 2007 > Application Management > Policy for Web Application (under Application Security). Select the Web Application you want to crawl and add the service account you are using for search. It only needs Full Read, not Full Control.