Can you change attack dice rolls from 1d20 to 1d100 or can you make it so that whenever you get a roll of 1 it should change it to a 2?
This is doable with nwnx, but changing 1d20 do 1d100 will need to be followed with a lot of rebalancing of all the other stats. However, nwnx2 (for 1.69) development is mostly dead, so if you lack the skills to do it yourself, you’ll need to motivate someone, likely from the nwnx:ee group. If anyone wants this for EE, they can just file a request at github.com/nwnxee/unified/issues
The rebalance would probably be too annoying. Can you instead just change the dice roll from 1 to 2 and 20 to 19 if you roll those numbers?
Yes (you could even change the distribution so that different numbers have different probabilities), but the nwnx limitations still exist - it is multiplayer only. In 1.69, there is nwncx, which works in singleplayer (windows only), but very few people are able to write extensions for it as it requires some files that bioware gave to select people under NDA. (and also requires reverse engineering, assembly and c++ knowledge)
I have disassembled other games before to change stuff I didn’t like, but I can’t figure out how to find the particular memory region that handles dice rolls in NWN.