you could be able to get nwriter to edit the ndless.cfg so you can easily change the File associations of ndless3 on-calc.
You already can do that. ndless.cfg.tns is nothing but a plaintextfile, therefore open it with nwriter change it, save it and you should be fine.
Note that I don't know how ndless handles this file, may be that you have to reboot your calculator for changes to take effect.
Also the default ndless.cfg.tns seems to use windows EOL while nwriter uses Unix style ones. This again may or may not be problematic depending on how ndless parses this file, in the worst case scenario you have to fix the EOLs via hexadecimal-input.