I have no idea, I don't have any useful experience using Codeblocks...
The fltk GUI designer is "fluid", and I just use it as a standalone tool. I guess it *might* be possible to ask CodeBlocks to launch fluid for you?
Certainly a lot of IDE (and indeed advanced editors these days) have mechanisms for adding the launching of external tools to the menus or whatever, so that might be all that is needed?
Of course, fluid does not directly generate C++ sources, it generate .fl files; these then need to be parsed into C++ by calling "fluid -c myfile.fl" to generate myfile.{cxx|h}
But I guess the IDE can do that too?
I usually just do all that in a Makefile, so I guess at a pinch CodeBlocks could run your Makefile too...