How to add emissive and base color

Hi everyone,

For a project to be used in Unreal Engine 5, I am asked to export my textures with the emissive only as a mask.

I have no trouble exporting the emissive as a grayscale mask, but the emissive colors are blacked out of the base color.
How do I keep the color of the emissive in my base color?
Is there a way to combine the two, base color and emissive color, on export?

Thanks!