.ksd files:
I was looking at Guitar Rig 4, and that is the file extension used for the presets. Maybe NI changed it for GR5?
My first method:
If you modify a preset then Export it, via the [+] button, as a .fxp preset, you can choose the folder to save it. Reaper keeps a note of that folder location and opens it automatically when you Export or Import another .fxp preset.
So you could build up your modified presets there. And use sub-folders too.
I've thought of something-else but this is painful. Get your friend to load each of the 400 or more GR presets and Export each one as an .fxp preset, into sub-folders. Then you can Import any of them when you need. You may need to buy your friend a meal to get him/her to do that for you.
I'm not 100% sure what you meant by "How should one use the other presets within reaper using the guitar rig 5's interface anyway?" There does not seem to be much keyboard navigation in GR.