For adding a new mesh that does not exists in the map you would need to force export that mesh with all of its dependencies(materials, textures, sockets...) into your target gmp.
Next step would be to create an actor with a component(s) and add it to the PersistenLevel's actors table. At this point your object should be visible.
Collisions are stored in the mesh itself so it should not be a problem unless you import something completely new.
Worst case scenario would be to generate kDOPs or RB_BodySetup for your new mesh.
Here is an example of BodySetup:
Moving objects around is the most simple part. You just need to edit it's position property. Since collisions are bind to the mesh there is no need to update them.
To get perfect results you would also need to somehow modify level's lightmaps and generate new shadowmaps for your mesh.
All of this is possible but will require a lot of time to implement and keeping package structure intact is pain in the butt.
RE should handle simple map edits unless there is a Terrain in the package. In this case offsets will be broken and Tera will crash.
Right now I am focused on exporting levels to UE4 as close as possible to the original, so gmp cooking is not finished.
I tried to move away invisible collision walls around a level and RE did it perfectly but doing something more complex is risky.
BTW is there a tool that will export DC to xml format? I am looking for a list of PPMs for all levels and I don't thinks they are stored in packages.
