Zandronum

More specifically DooM's guns to HeXeN. I wouldn't know where to start.

You'll need a tutorial that fits it good http://gunlabs.blogspot.com/2011/01/wel ... rmory.html

Or just use Samsara as you can already be doom's guy in it in hexen http://zandronum.com/forum/showthread.php?tid=200

Edit : removed dot from first link at end to prevent invalid link

you can check out the Decorate code in zdoom wikia, it is written in decorate style so we can use them as a base. but you'll need to rip the sprites and sounds from the iwad too

NOTE: you also have to convert the sprites to PNG or another supported format when you change from an IWAD to another, dooms graphic use a palette that isnt the same in hexen, therefore it looks odd, hexen reads the graphics and finds the code on the palette... and messes the looks up :V

AFAIK, you don't have to copy DECORATE, as long as you use actor names to spawn weapons. You still have to copy graphics and sounds, though.