After you decompile ESF's aura, open its qc file. (aura.qc). You can open it with notepad or something. Then delete all the hitboxes, except the first one, "aura". When you do that, add this at the end of the qc. Adding this includes the aura's textures when compiling.
$texturegroup skinfamilies
{
{ "blueaura.bmp" "blueaura.bmp" }
{ "greenaura.bmp" "greenaura.bmp" }
{ "purpleaura.bmp" "purpleaura.bmp" }
{ "redaura.bmp" "redaura.bmp" }
{ "yellowaura.bmp" "yellowaura.bmp" }
}
After you do that, say yes when it asks to save. Then import the aura you want, to ESF aura and scale it so it matches ESF's. After you resized it, select the new aura's mesh and assign it to the "aura" bone on the joints tab. You have to do this to both the "AuraUp" and "AuraDown" SMDs, or else youll get two different aura submodels in the same model. Then all you should have to do is compile it.
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I assume you are somewhat familiar with modeling and assigning vertexes, so I didnt go too indepth in explaining how to do this.
Nuttzy: There isnt anything on auras specifically, so I left the thread open.