yes you can.
aura is located in data/texture/effects/ in your sdata.grf file (assuming your using sakray)
you have to edit/replace
pikapika2.bmp
ring_blue.tga
whitelight.tga
freezing_circle.bmp
..and all you need now is know how to edit .tga's and/or find a site that hosts custom aura packs. good luck
also; instead of editing the .grf you can create a folder called
Gravity/RO/data/texture/effect/
this folder has a higher priority than the sdata.grf meaning if i have for example a custom pikpika2.bmp in my folder, Ragnarok will use that instead of the one in the sdata.grf