It would be fairly easy to implement if the object files were added to a specific directory where the game would search for additional items to the preset ones the game has (ie: \rising world\mods\). All that is required is the correct format of the items that I would presume would normally be a default of the standard design package, I doubt the devs would create more work than was necessary so the design packages default format is usually used in game objects. The game would not load incorrect formats and most files have an internal format identifier even when the file's extension has been changed.
Clothing for example, is usually a mesh and a DDE file from a standard 3D design package. If the code would read the games own files and then look in the mods\clothing directory after to see if there are any further designs to include, it would allow modders to simply add clothing items to the directory.
It isn't even an API and would only require minmal coding by the devs to achieve.
It would be down to the modders themselves to ensure the items they design have no clipping or other issues prior to releasing their mod. This is not something that later updates are likely to interfere with and therefore, modders would be unlikely to have issues that require frequent updates to the modded files.
Since we can only clothe the NPC's we spawn in clothes identical to our character currently, it would be nice to have a lot more diversity in clothing. Even later when we can possibly change their clothes, it would be nice to have a lot more diversity.
Rather than tie up devs with the mundane task of creating clothing items that may not be what players really want to see, why not leave it to the modding community? Modders could create themed sets of male and female clothing like those from a specific Western period like Victorian, 60's, 70's and so on. Also styles that come from specific countries like Japan, China, India and the like from differing periods in their histories from feudal to modern.
Rising WORLD can be exactly what it suggests.
In addition, various NPC characters could also be added in the same way, using the standard format the player character uses. Again, specific regional characters, Japanese, Chinese and so on that fit in with the theme the server has.