Can CKFinder be retro fitted.

There are no stupid questions

If you had used FCKeditor built-in file manager before, you may safely replace it with CKFinder. The only issue that you may notice is that FCKeditor used folders: "file", "image" ,"flash" ,"media" and in CKFinder default resource types point into different folders: "files", "images", "flash" (two folder have "s" at the end).
So.. after you point CKFinder into the right userfiles folder in the configuration file, the second step is to adjust the resource type configuration, this way CKFinder willl use the old folders (you can also create new resource type pointing to the "media" folder if you have used it).

CKFinder integration with FCKeditor is documented here:
http://docs.fckeditor.net/CKFinder/Deve ... ntegration
Resource types are explained in the configuration section of the documentation