Now you can either use this with an NPC tool or Wordpad. I often use the NPC Tool but the process is more or less the same with word pad.
So once you have extracted the stormtrooper npc, from the assets pk3, open it in word pad.
Once you have done that locate the text section called "Playermodel".
Where the tag says "stormtrooper" change that to the name of the EP7 Stormtrooper, which in this case would be "DT_Stormtrooper_EP7".
After you have saved your changes, you can either put the npc in a pk3, which is what I often do, or you should be able to just place in the base folder, provided you have made an ext_data/npcs directory.
Barricade24's post in Any way I can make the EP7 Stormtrooper Replace all Stormtroopers? was marked as the answer