It depends?
Not sure what your mod is about, but you usually don't need to copy and use the whole srb2.pk3
if you only plan to, say, just change the palette in PLAYPAL
.
The game loads srb2.pk3
and four/five other internal WADs by default.
Add-ons (or mods) add onto (or modify) these resources, replacing/modifying them or adding new behavior.