You have to have a package to at least tie back to a datasource, but writing direct SQL will not use the package in any other way.
I'm not saying you cannot do it, I am saying you _should_ not do it. Using SQL directly in a report bypasses tons of functionality built-in to Cognos, makes it non-portable (e.g. even database version upgrades could break it), complicates efforts at Cognos performance optimization, and short of going into each and every report, makes reflecting changes from your database to existing reports next to impossible.
Just say no.