For the first one: A lot of the addons don't add the Look as a custom Appearance or Preset, just a scene. So you'll need to load the scene, go to Edit > Person Atom > Appearance General (iirc), then Save that as a custom one. Then you can load it in the same way. For overwriting characters in other scenes, you'll want to only tick Appearance when loading - selecting Physical seems to overwrite Plugins, and Pose will move them.So I recently started getting more in-depth with this game but I'm having some trouble with two main things.
First is I can't seem to load characters into scenes. I can load their preview scene fine from the scene browser but within another scene I can't load them. When I go to load a look the only options I'm given are a bunch of the default stuff and 2 or 3 of the actual custom ones I have. I tried loading Demi (from Subverse) into the scenes but it doesn't load her properly.
The other issue is the animations don't seem to work properly in a lot of the pre-made scenes. The guy's or girls' hips/head won't be aligned properly so he'll kinda just hump the air or rub his dick on her body. I've tried adjusting them but I don't see how to change it permanently, going into play mode will have them go back to the default positions.
Demi for me doesn't load the hair - it looks like the Appearance was either made without it, or it's an object in the world that is attached after. I don't know how to duplicate that, but other addon models have the same thing - Madame M, Poison, Vora's equipment...
Any particular scenes for the second? Reign/Kitty/Zen Mocap stuff seems fine for me. You might be able to fix it by applying the DiviningRod plugin to the male - it'll re-orient the dick to work properly, but won't fix the whole pose.