I don't know whether Bullet do this. If Bullet don't do this, you can do it yourself. There is not much difficulty.
you can google the format for these file formats.
.obj file is based on string, it is not hard to load a .obj file as long as you know its file format and familiar with C file I/O function. As to .b3d, I don't know much, but I think if you understand its file format, it is also no difficulty.
There is a book on 3D model named 'Focus On 3D Models', you can find more idea from it.