Yes, you need a special tool made for KeeperFX.
You can compile it from KeeperFX sources, or get it from archive below:
keeperfx_tools.7z
If I remember correctly, that archive has password on it, and it's "keeper".
You only need po2ngdat.exe and char_encoding_tbl_*.txt.
The sndbanker.exe tool isn't finished - I don't think it actually works.
The other tools are for creating graphics files.