1) Sex Packs: These appear to be part of the files that get verified every time the software runs. If there's any way for users to create and share new sex packs, it will be news to me. Stuff that appears to be exclusive to "Sex Packs" include locations, new customization options and new toys (except for ones having the simplest geometry -- more on this below.) Things like the artwork for the tattoo packs and the jewelry add-ons may also be in this category.
2) Models: You can clone the existing models, make extensive modifications to them, and save your creations. The resulting files end up in Program Files\thriXXX\3DKink\Save\Models. I suspect, but have not tested, that these can be shared between users. Feel free to try/do that here. I'm especially interested in whether a set of Body and FaceMaker changes can be shared together so that users can effectively swap model designs.
3) Poses: You can save poses and find the save files in Program Files\thriXXX\3DKink\Save\. I assume, but have not tested, that these can be shared between users. Feel free to try/do that here. There's also apparently some provision for importing poses and motion capture data into the Pose Editor when building animations:
Poser Editor allows you to also import poses from the Gamerotica community or from Daz3D/Poser based BVH exports. Using standards based BVH allows you to quickly import existing animations and convert them to 3D Kink and create and watch some of the most advanced sex animations possible.
4) Toys: There's a "Toy Editor" with load/save function that lets you modify two basic shapes (cylinder and dildo) in some fairly complex ways, but nothing that departs from the fundamental topography of the shapes you start with. There are load/save buttons and it looks like these edits could be shared. It's unclear whether it would be possible for users to add more "Base Shapes" to this editor.
5) Movies: What about movies? The game lets you record them and play them back, but playback is very limited. The movie files are on your hard drive in Program Files\thriXXX\3DKink\Movies in a .gvc format. Presumably these can be shared if desired (I've not yet set up the forums to allow this upload format, let me know if it is wanted). If anybody learns how to convert .gvc format into something a little bit less proprietary, that would be useful info.
That's enough to be getting started with. Each of these is probably worth several topics of its own, so I suggest that this topic be reserved for identifying other specific challenges in customization and sharing. Answers and info on how to do it should go in custom topics, as specific as possible.