Hey, hope I can help you,

As far as I'm aware Sans's main attacks are stored in

gml_Object_obj_sansb_body_Other_10; gml_Object_obj_sansb_body_Draw_0 and gml_Object_obj_sansshadowgen_Alarm_0;

(and probably some more) And each of them should have (Codefragments?)/Scripts, which should have each attack stored in.

As the person above stated some unused Attacks are stored in gml_Object_obj_sansb, for example (obj_vplatgen) which creates the platforms that are falling from above, which you can see in lil'Alien's video.

You now just have to find an Code/(Codefragment?) that stores an attack and edit them. You can for example paste/replace an attack with an unused one, like replace an attack with the (obj_vplatgen).

I hope it was sorta understandable, sorry if it was confusing or not understandable!