I don't think UE uses it's own editor to model/UV-map/texture-paint the assets. More likely, it's capable of importing meshes (with joints, skinning etc) created in other 3D-packages (like Max or Maya) and exported as FBX.
What causes the problems for us is that UE doesn't keep the meshes in that format (FBX) and converts them to it's own. So unless you have the sources of the game or a tool that can extract these meshes and convert them back to FBX, you won't be able to modify any of the assets.
For now, all we need to do to make things get moving is find a modder that has experience with
any game made at UE. I'm pretty sure there are modding tools for UE out there. I just don't know anything about them. Because I haven't ever worked in UE. So we need to find the one who had.
Although I can try to do it by myself, I'm too busy now to spend extra time for getting familiar with new software. So If I try to do it on my own, I'll spend weeks or even months before I get familiar with UE good enough to be able to create entire mod.
UPD: But if we (I mean, all the visitors of this topic) won't find experienced UE modder for the next month or two, looks like it's time for me to finally meet with UDK/UE.
Like they say, if you want to get the work done - do it yourself.