This is a site where you can change textures, move them, and make them semi-transparent.
We'll use this to move this.
Import what you exported with VRoidstudio.
Open the image list and display the material of the umbrella hiding the butterfly.
Export the displayed material and rewrite it in a paint software.
Leave the color as is, but repaint it so that it hides the top and bottom of the wings alternately when it moves.
(This may be hard to understand in words, so download my bag from BOOTH for reference.
This is material [19].)
Add a new material.
Scroll down the material item and you'll see the UVAnim column. Move the bar to move the material.
The maximum movement speed when operating the bar is 1, but you can enter a number to make it faster. My bag is 3.
Did it flap its wings?
If it was successful, you can export it and use it.