Is there a way to add the alien loot to already spawned biters? I tried the console command to kill all biters, but the newly spawned biters did not have the loot either.

Biters have loot. From ore 1 at the first levels, to ore 3 and artifacts at 10th. This does not depend on the time and method of spawning biters.
Check in the editor (place some turrets and biters)
If not, then I'll need your save.

I placed some enemy biters and killed them and there spawners, no loot, unfortunately. I know how to get to the save file in appdata, but how do I share it with you?

On the vanilla hives/biters it says Basemod>Alienloot economy > Rampant fixed> combat mechanics overhaul, but on the new biter/hive types it says Basemod> combat mechanics overhaul with no mention of alien loot.

The save can be shared, for example, through Google or yandex file hosting.
Here's an example test. On this save, biters drop loot (settings have been changed so that corpses disappear immediately)
https://disk.yandex.ru/d/mce2Qaj-CPNm9w

Thank you, this is our save file.

https://drive.google.com/file/d/1geIciDvQTc4KzeevhgmaHqWG_QQOf9EL/view?usp=share_link

Reason: Rampant Fixed is compatible with the SchallAlienLoot mod.
You are playing with the "alien-module" mod (aka Alien Loot Economy).

They are different mods. I will add this mod too, but don't wait until a week later

Oh, sorry for the mix-up thank you again, and thanks for the great mods!

You can use the rampant-alienmodule-compat mod and edit its dependency so that it optionally depends on both Rampant and Rampant fixed, so that it can apply its patch regardless of which Rampant you have.