Blend is for 3d model, while Jbeam doesn't contain 3d model data. You can't change a blend to Jbeam, it's like converting a text file into an mp3. Read the wiki if you don't know what a Jbeam file is.
A .blend file contains 3d model data in a format proprietary to blender (a 3d modelling program). BeamNG loads a single .dae file for its model data and a selection of JBeam files for physics data, a .blend containing just an engine is entirely useless to it. In order to use it, you would have to import the .dae for an existing car into blender, import your .blend file in, fit the scaling of the engine to fit the car, reset all the transformations, redo the normal mapping, UV map the thing, export to .dae and finally edit the JBeam files to adjust to the new object being present in the mesh. We are getting multiple DAE support soon. This would streamline the process a little. You would still have to scale the engine to fit the car, do all the normal mapping, transforms, UV's and JBeam etc, but it wouldnt have to be done on an imported version of the original car, it could be exported as its own .dae which the game loads separately. As it stands. Huge amounts of work and from your post history I am going to go out on a whim and say impossible for you to do yourself (please don't take that as an insult, at near midnight when I want to go to bed its hard to think of politer ways to put things, the exact same result goes for 99% of people on this forum too)