Unfortunately both Gen IV Sprite Editor and PokeDSPic seem to not be able to edit the alternate forms because they only edit the pl_pokegra.narc file. Alternate forms are in pl_otherpoke.narc.
I would love to know if there's an editor for this cause I haven't found one. What I've been doing to edit the alt forms is very improvised but might help you so here it goes:
1) Make a copy of your pl_pokegra.narc file
2) Open the copy in the sprite editor and replace Arceus' sprites with the ones you want (technically you could replace any pokemon but I find Arceus to be easier since they're genderless and the last one in the list)
3) Open your ROM on Tinke (or whatever program you use as long as it's capable of extracting and replacing files, and also unpacking narc files)
4) Replace the ROM's pl_pokegra with the copy you just edited, just like you do when adding regular sprites
5) Now unpack pl_pokegra.narc. Almost 3000 files will show up. Scroll all the way down to the last ones, which used to be Arceus but are now the sprites you inserted. pl_pokegra_2959.RGCN is the back sprite, pl_pokegra_2961 the front sprite, pl_pokegra_2962.RLCN the normal palette and pl_pokegra_2963.RLCN the shiny palette. Extract these four files.
6) Now unpack pl_otherpoke.narc (it should be right above pl_pokegra.narc). Around 250 files will appear, they are the sprites and palettes of pokemon with alternate forms, and you can replace the ones you want with the files you've just extracted.
Of course, the problem is finding them since the list numbers don't correspond to the pokedex. They're still roughly in pokedex order though, I haven't tested all of these but here's how it should go:
0-7: Deoxys (normal form back, normal form front, attack form back, attack form front, defense form back, defense form front, speed form back, speed form front)
8-63: Unown (in alphabetical other, as usual backsprite first then frontsprite)
64-71: Castform (normal form back, normal form front, rain form back, rain form back, sun form back, sun form front, hail form back, hail form front)
72-77: Burmy (grass form back, grass form front, sand form back, sand form front, trash form back, trash form front)
78-83: Wormadam, same order as Burmy's sprites
84-87: Shellos (west form back, west form front, east form back, east form front)
88-91: Gastrodon, same order as Shellos' sprites
92-95: Cherrim (closed form back, closed form front, blossom form back, blossom form front)
96-97: Arceus (normal form back, normal form front). I actually have just replaced this in a hack of mine so these are the ones I'm sure of the most.
98-131: the other Arceus forms in the same back-front order, but I haven't yet tested which type is which.
132: empty
133: Manaphy Egg
134-137: Shaymin (land form back, land form front, sky form back, sky form front)
138-149: Rotom forms, I haven't checked the order yet
150-153: Giratina (altered form back, altered form front, origin form back, origin form front)
From 154 on are the palettes in similar order. It's easier to find these since you can view them on Tinke:
154-155 = Deoxys (normal and shiny)
156-157 = Unown (normal and shiny)
158-165 = Castform (normal, sun, rain, hail, and then shinies in the same order. the colors are exactly the same for the weather forms, so I don't know if they're capable of being shiny or not)
166-171 = Burmy (grass, grass shiny, sand, sand shiny, trash, trash shiny)
172-177 = Wormadam (grass, grass shiny, sand, sand shiny, trash, trash shiny)
178-181 = Shellos (west/pink, west shiny, east, east shiny)
182-185 = Gastrodon (west/pink, west shiny, east, east shiny)
186-189 = Cherrim (closed, blossom, closed shiny, blossom shiny)
190-191 = Arceus Normal (normal and shiny)
aaand I haven't seen the rest yet, sorry. But it should be the same pokemon order as the sprites, alternating normal and shiny palettes.
7) Once you have replaced what you wanted, pack pl_otherpoke.narc
8) If you want, you can replace pl_pokegra.narc with your original file to undo what you did to Arceus, although to my knowledge Arceus' sprites in pl_pokegra go unused anyway so it might make no difference.
9) Save the ROM, you're done.
Hope this helps! It's really coincidental to see someone asking this when I found out about pl_otherpoke like, last week.