YUP. I just did it. You can just use notepad. Its not easy at all. For example, I have Cakewalk Studio and it comes with Studio Instruments Drummer. They have a bunch of kits and there is not a kit that has the collection of bass, snare and other things i want. A kit for the drums really is a PROG file. The prog file is a collection of SFZ files. Its a weird format, but it is possible to change it. I found the text where it points to the Kit 1 Kick drum.sfz. I went into notepad, changed it to Kit 2 Kick drum.sfz and it worked. Now that PROG points to a different SFZ file. So, its possible. I have not tried to make a bigger change than this yet. There may be issues with spaces and file lengths.
The way the Studio Instruments Drummer is set up in the prog file, it has 8 groups of sounds. The kick drum sounds is an sfz that has more than one sound in there since there are two midi notes that trigger the two sounds. So its a structure that you cant mess with much, but if all you want to do is use one sfz over another, it looks like its possible to slave over the text file and get it to make a kit that it never had.
I am still amazed that these software makers dont do the simplest thing you would want, which is to simply pick what sound each note triggers. For drums, you are going to want to pick each sound. You are not going to want to be limited to the kits they set up. You have to go up to the next level Session Drummer 3 in the Producer edition to do that. Basically, to change a text file, you need to get to a different level. Its like they dont even want you to use the sounds they gave you. Nutty. Who comes up with this stuff.
I can just see the developers saying - lets make a drum machine and we tell them how to assemble their kits...when they get it, they will be switching back and forth forever listening to all the junk in each kit and be forced to pick from whatever we assembled across all the kits....they wont be able to pick each instrument and get what they want...its genious.... what bs. How hard would it be to pick a different sfz and save it, come on.