That task is one of the messiest. This is why OpenBRF embeds a mechanism to do exactly that.
It has been a long while since I programmed it in or used it, but this how it works:
Step 1: Activate the mode:
Under settings, select "on assemble vetex ani" => "quiver mode (start with max arrows)"
Step 2: Prepare your meshes: the one with all the arrows, the one with all the arrows but one, etc.
You can import them one by one, or you can do this in OpenBRF itself if you want:
import your most complete quiver, split it in connected subcomponent, then assembles these components back into frames.
It takes a bit of patience.
Step 3: Start with the most complete frame, select next frame (where one arrow is missing), copy it (ctrl+C), "paste frame" (under edit) to the animation. Select next frame (where one more arrow is missing), repeat.
Step 4: Revise your animation: the best way is to select your final animation alone, then do a "split all frames". This will produce one mesh for each frame. Double check them, rearrange them in the right order (which is: "full quiver", "full quiver" again, "empty quiver", "quiver with 1 arrow", "quiver with 2 arrows"... , "quiver with N arrows (full quiver again)") . Then select them all, and"assemble meshes into one vertex animation". Then fix the timings (that's what the game engine uses to determine which quiver will be shown when there are N arrows left).
What is happening in step 3: every time a frame is stacked, the 3D pieces which are present in the frame 0 of the receiving animation, but are absent from the frame being added, are added to that frame but collapsed to a point. This is because the mesh must technically be there, but must be invisible.
PS: I could do a better tool to do that, one which is considerably simpler to use. How much and to how many modders would that be useful?