how to map fields in feeds module with csv that has no headers? Can feeds module still know how to map the fields correctly even if there are no headers in a .csv file. I am trying to migrate sample data from database which i exported to a .csv file through phpmyadmin. i am testing the feeds module if it can handle large file migrate.

if the answer is no, can i manually input the headers in the excel files? or it has to be exported again with headers?
The Feeds module support CSV files without column headers.  On admin/structure/feeds/foo_importer/settings/FeedsCSVParser, you just need to select "No Headers".  Then on admin/structure/feeds/foo_importer/mapping, you use the column numbers for the source.

Be careful when entering the Name of source field. Your first column is named '0', the second '1' and so on.