You can import terrain data into the toolset to help build exterior areas using YATT. From the YATT website:
Yet Another Terrain Tool (YATT) is a plugin for the Neverwinter Nights 2 Toolset allowing importing of terrain information (including heightmaps, colourmaps, texturemaps, etc) directly into the toolset. It does the importation of height, colour and texture data directly through the TRN stream, meaning it's reasonably nice and fast.
You can download the YATT plug-in form the vault. There's also a good tutorial on the vault for beginners by Laban. The YATT website also has a number of case studies and additional information.
One way that some builders generate terrain data is with L3DT. From the site:
L3DT is a Windows application for generating artificial terrain maps and textures. It is primarily intended for game developers seeking to make large high-quality 3D worlds (eg. for RTSs, MMORPGs, etc), as well as for digital artists who have designs for their landscapes and need a program to build them.
Thanks to nicethugbert for point me in the direction of some of this (I read his sig on the forums). I've known about YATT for a while, but didn't have much luck with other mapping programs. L3DT seems to be more promising. Also check out NTB's Eroded Hills and Valleys, where he has posted some of the areas he generated with these two tools.