As i know, the Hexen Expansion set can be played only using it's IWAD.
In the case you want to run the game with a command line (MS-DoS/Linux Shell), you have to use the "-file" option, instead of "-iwaddir". That means that you will be using the Original Hexen IWad as main WAD, and the Expansion WAD as a PWAD.
But if you are on a Windows OS, you may use the Vavoom Launcher (vlaunch.exe) or this other Launcher, designed for Vavoom too (i recommend it): <!-- s8) --><img src="{SMILIES_PATH}/icon_cool.gif" alt="8)" title="Cool" /><!-- s8) -->
[url:jm7pvodm]http://vavoom-engine.com/forums/viewtopic.php?f=10&t=1493