Blender version 2.49b has tools which allow you to import .OBJ files and export .NIF files, the file format that Fallout 3/NV uses for its meshes. Links to blender 2.49b, the niftools, and python required as well as install instructions can be found here:
http://niftools.sourceforge.net/wiki/Blender
You will also need nifskope to clean up the .NIF files found here:
http://niftools.sourceforge.net/wiki/NifSkope
From there you would want to rig your 3d models onto the humanoid skeleton used in fallout 3. There are few tutorials about this:
http://wiki.tesnexus.com/index.php/Creating_an_armour_for_Fallout._Part_1