How can I include custom tables into search results.
My all custom tables has qa_ prefix and wordpress has default wp_ prefix
It's probably SQL Full Text Search what you're looking for. Maybe this tutorial helps you forward: http://www.blrf.net/howto/49_Wordpress__How_to_implement_MySQL_full_text_search_.html
There is also a WP plugin with Full Text Search capability, but it's not updated for a while: http://urbangiraffe.com/plugins/search-unleashed/