Unfortunately, Opus can't do this, at least not directly.
If you are searching text files then you might find the Grep Dir button in my Diff/Merge Toolbar useful. It searches for strings in all files and subdirectories and prints the matching lines & file names.
It's not pretty but I've found it good enough for quick searches, and I load a text editor and use it's Find in Files feature when things get more complex.
Don't forget to send GPSoftware a request if you'd like to see something better integrated into Opus.