Is adding a new object into an old UV map possible?

Basically what the title says. For example, let’s say that I have a cube, and I select all of it in edit mode and smart UV project it. And then I arrange the UV map into a formation that I like. Then after a while, I for whatever reason, need to add in a object while in edit mode of the cube.

But the problem is that the new object’s UV map will not be in the original cube’s UV map. Is there a way to bring the new stuff in the old map somehow? It will save me hours of work if so!

EDIT: Ehh, never mind I guess. I think I found a way around it. I just selected the new faces and unwrapped them, and then selected all of the faces and somehow it came onto the old map.