The modifier key is usually called option on the Mac, but I assume you're talking about the ability to select in a "box" rather than full lines. Is that right?
If so, I believe it's pretty much up to the application developer whether they chose to take advantage of this feature or not. So no, so far as I know, there isn't a way to enable it on apps that don't support it (though with X11 apps I'd expect at least some to have the feature available, if not enabled by default).