I just downloaded the "whacked" program. I know that the 128kb wad has custom enemies using dehacked. I did an experiment to see if I could replace worthless sprites (such as there being two blood and guts sprites rather than 1) with an used sprite in the doom iwad. When I try, the program changes the sprite with both blood and guts (id #10 & #12) things at once, rather than just one of them. In the 128kb wad, just one of those files was replaced with a custom enemy. I also tried to replace the "blocking" versions of the hanging corpses, in this case the "hanging leg" w/ "blocking" with the unused stalactite from the iwad. This also replaced both sprites, one of them worked while the other hanged from the ceiling, because they were placed in different locations. Is it possible to replace actors that are used more than once?
example:
https://doom.fandom.com/wiki/Unused_Graphics_and_Objects?file=Doom32x_-_Small_Grey_Stalagnite.png