Can goldsmiths modify rings? or do they usually only work from scratch for custom work?

I’m starting to think seriously about engagement rings and I was hoping to do something a bit custom. I’ve found a couple of choices, but I was hoping it might be possible to swap out the stone, and possibly add others to it. I expect a true custom ring to be prohibitively expensive, but would it be that hard to do some small additions to an existing ring? or is that not possible/worth their while?

Does anyone have any experience with custom rings?