I've got the addon's working by placing both duke3d.grp and the addon .grp e.g. nwinter.grp inside the /3DS/eDuke3d* folder.
*using the .3dsx version
Edit the eduke32.cfg file, and look for:
SelectedGRP = "duke3d.grp"
Change it to the addon .grp
e.g.:
dukedc.grp
nwinter.grp
vacation.grp
All seem to work fine.
The main menu of the 'Life's a beach' addon still displays 'Atomic Edition', but in-game all sprites and sounds are loaded correctly and the game runs fine.
This way only one addon can be loaded at a time so i created multiple folders:
/3DS/Duke3D
/3DS/DukeDC
/3DS/DukeNW
/3DS/DukeVC
Each including the duke3d.grp and their respective addon.grp and a modified .smdh.
The duke3d.grp included in the download does NOT work with addons!
It is an early shareware version, use a v1.5 version of the the game.
I've used the Megaton Edition duke3d.grp [v1.5]
Could you share the smdh files please?