Thanks for catching this. I'll hopefully have some time over the holidays to look into the issue.
Edit: I've now turned on the ticket tracker for BunnyCensor, and added this as ticket #1.
I'd love to give people the flexibility to add their own words. If you are comfortable with mucking about in code, you can do so by editing BunnyCensor.lua. See Wis2Wiz2's post below for an example.
With that said, I'd love to have a command syntax to the effect of:
/BunnyCensor add foo
If anyone knows how to build a case-insensitive lua-compatible regex from that, please let me know, or contact me about joining the project