I have a site on localhost:81 and extended it to localhost:82. I'm able to crawl the 81 but i'm unable to crawl the 82.
Can we crawl an extended site in SharePoint?
The extended site should be already considered if SP is crawling the default site, like djeeg mentioned. An extended site, in most cases, is just an alternate method to access the same data, so it wouldn't need to be indexed separately.
What results are you seeing?
SharePoint crawls the default zone. If you want it to crawl the extended web application (82), make it the default zone.