Correct, No API's available to get and therefore modify the SQL in a SQLExpression.
Only option is to use a database Command Object if you must modify SQL.
And to explain more the reason is the data from the SQLExpression is heavily tied into the report itself and therefore making changes to it can have serious effects on the linking and other parts of the report.
There is no SQLExpression Controller so .Modify will not work, requires RAS to be able to make changes.
Don