What you need:

-The wad from the full version of Hexen (shareware not supported natively by code released by Raven, may be integrated in later versions).
-A flashcart big enough to hold the binary with wad (well over 128mbit).
-gbfs by Damian Yerrick for appending the wad to the HexenDS binary. You can obtain it here: http://www.pineight.com/gba/
-A way of getting your DS to boot off of said flash cart, which also allows the arm9 code to access the cart's rom. I personally use wmb and a F2A 256mbit cart.

How To Use

-Place your full version Hexen wad in the bin directory where makewadgbfs.bat is, along with the ds and ds.gba files. Also make sure you have gbfs, lsgbfs, padbin, and cat somewhere on your path.
-After fulfilling the above requirements run makewadgbfs.bat, if all goes well you will end up with hexends_wad.nds and hexends_wad.nds.gba files.
-Flash the .gba file to your flash cart, use the method of your choice to get your DS to run arm9 code off of the cart. This method must support access of the flash cart's rom section or HexenDS will not run.