How do I convert BoundingBox values of a billboard sprite on a 3D layer to a 2D layer?

Doesn't BBox calculate in X/Y only? I am assuming in your case the vertical axis is Z?

If your objects are square/circular, I would just get the center in screen coordinates (using LayerToCanvas) and calculate the width/height using the distance of the billboard to the camera plane (as we know at that distance, the Width/Height are exactly 1:1 with the width//height on screen).

Alright, I figured this out! Funky Koval I wasn't sure how to even approach the suggestion you put forth (thank you all the same) and I was concerned that it wouldn't easily allow for things like Z axis rotation being reflected on the collision checking sprites.

Ultimately what I did was add 4 image points to the Billboard Sprites on the 3D layer that correspond to their top, bottom, left, and right sides (effectively recreating bounding boxes)

Then after spawning the collision checking sprites on their 2D layer at the 3DCamera.LayerToLayer positions, I set their size using the distance from the image points on the 3D layer:

Finally, if you want to set the angle of the 2D collision checker to the billboard sprites Offset Z, you do so with those same imagepoints:

Ultimately I am going to try using this to create a pseudo-3D collision checking system for attacks/enemies. Attacks will first need to collide on the 3D layer, then the game will run this separate check to see if they collide on screen.

This approach works in this dungeon crawler view where the camera is going to be pretty static during combat, I don't know how well it would work elsewhere.

Yes, the 4-point approach would be what I would use if the shapes were not square or billboards. This is absolutely valid and (as you've noticed) accounts for rotation and even mesh distortion. Glad to see you've figured it out!