I found this source code that contains some tools to manipulate the spt files from The Elder Scrolls IV: Oblivion. I managed to compile it and they seem to work with the spt files from NWN2.
The tools are rudimentary. One tool converts a spt file to a text file and another tool converts the text file back to spt. It’s very complex to modify the text file since it contains more than 200 parameters describing a tree.
Here are some tests. This is the original PinOakF.spt:
Here I have lowered the branch frequency of the trunk:
Here I have ridicously increased the flexibility of the trunk, so it sways