I've downloaded some outrageous clothes as well as some base game compatible clothes from the Movie Stuff Pack so my Sims could wear them as costumes at costume parties or when going trick or treating but the game always makes them change into the default in-game costumes. How can I edit the file in S3PE so that the game will recognize it as a costume?

(Sorry for digging this up)

It's actually done with a XML. With the: TrickOrTreatSituation_0x89121649c768f2d7

There is a line there that says:
<kChildCostumeNames value="cuBodyEP8Costume1, cuBodyEP8Costume2, cuBodyEP8Costume3, cuBodyEP8Costume4, cuBodyEP8Costume5, cuBodyEP8Costume6, cuBodyEP8Costume7, cuBodyEP8Costume8, cuBodyEP8Costume9, cuBodyEP8Costume10, cuBodyEP8Costume11, cuBodyEP8Costume12, cuBodyEP8Costume13, cuBodyEP8Costume14, cuBodyEP8Costume15, cuBodyEP8Costume16, cuBodyEP8Costume17, cuBodyEP8Costume18, cuBodyEP8Costume19, cuBodyEP8Costume20">
<!--Child costume package names. Verify costumes work before checking in additions.-->
</kChildCostumeNames>

(And many more, same for elders, adults, teens, etc.)
They share the name of the clothing. I have no idea what the name is for the movie stuff packs but since it's made by EA, it's easy to check by opening the package and see the name that all files in there share. Then, copy that line and add it to that XML line. Though I think the 1>20 at the end of the names are either different colours of the costumes or actual names.

Thanks so much, Greenplumbbob! You're such a helpful member of the Sims 3 community!

I'll start up my computer now to look at the xml file. Do you know if the TrickorTreatSituation covers costume parties too? I assume so since you mentioned there's code in there elders and adults. Also for adding cc clothes to the list, how would I go about it if it doesn't have a name?

You can trick it with S3OC, open S3OC, set your CC installation folder to S3OC, clone the ones you want to add from your CC, check all "Deep Clone", "Create Missing String tables", "include thumbnails" and check the "Keep Original STBL IIDs" boxes when cloning, once it's done, open the cloned (package file) with S3PE, in some cases it will show you what its original names..

But wouldn't that add the original outfit the cc was cloned from and not the cc?

Yes that was the purpose, to know every details where the cloned one was originally from, if you open S3OC and click "settings" and then "Game Folders" and enabled the CC folder installation folder (as well as the EPs and SPs installation folders), that would cloned from the CC, as the ones you're trying to modified, if it has names from its original state it would also be in the cloned ones. It's like if or when you open any package file with S3PE, and then you copy one (or few of the contents) with ctrl+shift+C key and then paste them into a new blank/empty package file in separated S3PE window, if you don't check the "rename if present" box, it won't display the original name(s).

The purpose of cloning the object with S3OC was only to find out what is the name of the content that you're looking for, you won't be needing the object that you had cloned in your purpose for re-categorize clothing as random costumes , once you found the name(s) you need, you can modify the XML you'd like to modify, and then you can delete the cloned ones when you don't need them anymore.

I looked them up to how EA did it with Seasons, and it seems that they're all more like 'merged together' outfits. So say a child sim wants to be an alien, the game will pick something like cuBodyEP8Costume8 (not that this is the alien line).

Now I've been trying to find the script for the Seasons costume but I can't find it. But I think it works the same like the fairy's wings:

{
"ep7SimFairyWings1_CAS",
"ep7SimFairyWings2_CAS",
"ep7SimFairyWings3_CAS",
"ep7SimFairyWings4_CAS",
"ep7SimFairyWings5_CAS",
"ep7SimFairyWings6_CAS"
};

I just need to see where the script for the trickortreat is and then I hope I can help you out adding it to the XML or maybe even need to make a whole new script. But i'll let you know if I found it unless someone else knows where to find the script?