You dont need to make the animations... while items are certainly not my field of expertise, i believe declaring an object as a certain (native type) weapon will automatically assign it the correct animations (that all items of that kind use).
Incorrect, I'm afraid. You must make a vertex animation.
What's a vertex animation? It's a series of "frames" that are the same mesh, but with changes to the mesh vertexes for each keyframe. Check out the Native bows; as you cycle through the vertex animation frames, you can see what's happening there at each frame.
For bows, it's very important that your bow be in the right place and is bent correctly to match Native bow positions, especially the strings, for each keyframe. Failing to do so will result in a bow that has the string "floating in the air" and looks really bad.
To export Native's bow animation frames, select each frame by cycling through the keyframes and right-click; there's a menu option there to export that frame of the vertex animation as a static mesh in the OBJ format. Then use those references to make changes to your own model and match the positions of the bow's handle and string at each frame, then export the changed version of the bow, import to OpenBRF, and assemble the vertex animation.
None of this is very hard, you just need to be patient and look carefully at what's required for each frame
