. I need to allow user to specify search value on a field which doesn't exist in search BO. Then in searching event, I would add new condition to WHERE clause based on this value. Is it possible? How?
Yup. Look at the advanced BrowseDialog sample and the docs in the 1.6.5 release to see how to handle the Searching event. Just create a custom field in the BO to which the search field is attached, then adjust the query through the Searching event of the BrowseDialog when executed.
I want to use custom control as search field input in browse dialog. Possible? How?
Not possible at present. We dynamically create the controls so that we know how to deal with them. So if you need to have custom controls as the search fields you may have to create your own browse. Sorry. 