How to put a dynamic 3D object into the game?

Those hammers are this mod:
https://www.beamng.com/threads/giant-sledgehammer.13548/

It's a 'Vehicle/Prop', not an animated objected exported from a 3D software.

 

If you are just starting modding, I suggest something easier.
Props are basically vehicles, except they can be anything you want them to be.

Props are objects that uses the same 'Node & Beam' structure as vehicles, therefore they can interact with vehicles in a dynamic way.
Opposite to 'Static Objects' (which is just a 3D model exported into a DAE/COLLADA format and imported in the game, which is fully static - vehicles can collide with them, but they cannot move, as they are static)

If you want to dive into Props, you need to learn how to Jbeam (ie. create Node & Beam structures). This is one of the guides:
https://wiki.beamng.com/Introduction_to_Vehicle_Creation