Yes, as long as they can export to .X format. DX8 should do fine
Just go put your .x file into a folder named what you want to name it, have a small thumbnail of it in the folder with the .X file and its textures (I can't recall the necessary dimensions for the thumbnail, look at the other model folders for them).
You plop your model into the Player or Object folder, the type folder (horror, space, etc.), any other folders that apply to your model, and then you can use it. But I never figured out how to get the animations working
And you also need another file, I can't remember what it's called, but it holds values for the object. You can edit them as you please, and I think they're pretty self-explanitory, and you can edit the file with any text-editor.