It's not easy at first and can get tedious, but, be warned! Tileset manipulation can become very addicting. I started adding rural raised terrain to the desert tileset in '04, I ended up making 3 tileset addons for the forest tileset. The third one I never quite finished, but weighed in at a pretty hefty 65+ megs. lol
It's a great skill to learn, as you are no longer at the mercy of other tileset modders who sometimes quit their projects before they add enough features or don't add features you like.
Here's some tools to make life easier:
NWN Explorer - for exporting your tiles, set and ITP (pallette) files
Tileset Duplicator - for copying and renaming your tiles
Set Editor - for editing set files... obviously. You can go 'old school' and use Notepad like I still do, but then... i'm an old curmudgeon who hates doing things the "new" and easy way. '>
TLK Edit - Yes, I know it's a "TLK" editor, but you can use it on ITP files too! It's the best tool for editing ITP's, IMO
Jackal