To "convert" apps to Cocoa, they'd have to be rewritten in Objective-C. At this point, you're totally rewriting the app. On top of that, X11 draws using bitmaps, while Quartz draws using PDF and PostScript. Raster vs. vector there. Yeah, that's not so hard to fix... but the Obj-C thing would require the whole thing be redone.
Most X11 apps are C++, so I guess it could be easier to rewrite for Carbon, as that's a C++ API. However...
Carbon carries a lot of legacy Macintosh baggage with it. It's the "modernized" version of the Mac Toolbox API, and therefore would require that certain bits be rewritten to interface with the API. Also, Carbon doesn't support certain things that aren't permitted in Mac OS 8.6/9.x as it was designed as a backward-compatible transition API.
In the end, the easiest and best way to run X11 apps is just to compile the thing for Darwin, and run it in X11
As for my earlier statement, it's not so much a dig on X11, but a statement of the usefulness of it on OS X. There's a lot of good software out there that can be run... but... there's also an equal amount of good software that runs natively on OS X. That's all I'm saying in that regard. If you wanna use UNIX apps, just go with Linux or BSD. No need to spend the $$ on a Mac. Or dual-boot X and Linux!