I know this is not what you asked for, but I have a possible workaround, depending on why you don't want to add mods/CC.
I don't think it is possible to access the hazmat suit in CAS without mods or CC. If you post in AHQ in the General Section, I know there are both builders and modders/mod users reading that part of the forum, that perhaps will know more.
What you can do, which I just now tested and confirmed works, is to use an unlock mod to access the hazmat suit in CAS for dressing the mannequin; and save that mannequin in a lot or room to your private Library or the communal Gallery; downloading it in the save you want the hazmats in, without having to add the mod in that folder.
The mod I found and used, was Fogity's Unlocked CAS Parts:
https://www.patreon.com/posts/mod-unlocked-cas-38063320
List of what is included here:
https://fogity.wordpress.com/mods/unlocked-cas-parts/package-list/
Method: I made a new The Sims 4 folder, added the mod part Gameplay; dressed the mannequins; saved them to Library and Gallery.
Created yet another The Sims 4 folder and exited the game; did not add the mod; copied the tray files from the first folder and pasted them into the Tray folder in the newly made TS4 folder. Started the game; downloaded the room from library, as well as from Gallery, onto two different lots. The hazmat suits were still on the mannequins, and fully functioning when making the lot a retail: my sim could try on the suits, buy them, and when owner of the retail, restock. There can just be one swatch per clothing category, but adding all the swatches (yellow; green; blue; grey; red) to different clothing categories makes them accessible through the option to 'Dressing mannequin'.
This method works, I think, because the mod only unlocks the access to the hazmats suits, which is hidden by default and probably intentionally so for the Strangerville narrative, by the game. But the mod doesn't add anything, and so once the suits are accessed and placed, it is not needed anymore for them to be where they were placed, whether this was on a mannequin, or on a sim.
If you need more directions please let me know. Sorry about a late answer to your question, I haven't had access to my gaming PC for awhile. Hope you see this though.