Alterations

Depending on the dress I think this is definitely possible. Your best bet would be to contact the seamstress you plan to use and ask them.

To be able to answer this question you’d need to share a pic of the dress, I think.
it’s totally possible to add layers to a dress, but it just depends on the silhouette of the dress and the way it’s constructed, as well as if you want the layer to be visible or not.
For example, my dress is a plain mikado silk ballgown. Not see through at all, so the extra layers of pink tulle I added are not visible through my skirt, and there was plenty of space for extra layers under there. But if I had a tulle skirt, you’d see the pink through it. And if I had a trumpet or mermaid style, there might not have been much room to add anything, so it just depends!

Like Rosie said, it all depends on the fabric! If it's tulle, and aline or ballgown, that's a super easy and straightforward alteration. Otherwise... Could still work, but might be more complicated. Would love to see a picture!

I think this might depend on style of dress and the reasoning. If it’s to add a little pop of color, it could probably be done fairly easily. If it is to serve as a nude-type of lining, it would likely be more complicated. Do you have a picture of the dress you have in mind or inspiration for how you’d like it to look?