Can you please explain in much more detail how you do that and on which versions of the game and of undertalemmodtool,because i've tried and i've run into errorsYeah, download "UndertaleModTool" and open the "data.win" file with it. The Tool should work with almost any Gamemaker Game. You will find all the CGs and such under the Sprites. You can also double click a sprite to open it inside the tool without the need to extract it first.
The CGs should have "scn" after the "spr_"
while the Idle Poses should have "Cutout", "char" or "idle" in the name. (Mostly)
And the main Poses are just the name of the character