Sprite packer with loaded resources

We are making a 2D game, and use the Asset loading mechanism to load "World"s from the Resources folder. We do that to manage memory and unload worlds when the user moves to another world. Every world consists of several sprites.

We want to reduce draw calls and use a texture atlas for each world. We use this for all our statically loaded sprites and we see great difference in the number of draw calls when using atlases.

What might be the way to achieve that with assets that are loaded dynamically?

The editor does not automatically packs those assets even when using the "Packing Tag" property.