Easy way is to create a new report with the same sql and during the wizard, there is an option for a search bar.
What you do is you create an item for example PX_SEARCH and then you create a search button that does nothing but to submit the page. That way you send the value of the item to the session after the user presses search.
Then you can use the item in the where clause of the SQL query of your classic report.
WHERE (mycolumn LIKE '%'||:PX_SEARCH||'%' OR :PX_SEARCH IS NULL)