You go back to your other 3D app (Maya, 3DS Max, etc) and you re-export your whole scene with the new object added to it (I strongly advise FBX for many reasons). Whether the new mesh you want to add to it shares the same UVs and/or textures doesn't matter. The two others are damn useful in my opinion.ġ/ You already have some hand-painting done but you need to add a new object to an exisiting Substance Painter Project. There you can reload the mesh(es) from your scene in progress. Three great ways to use what you'll find in Substance Painter's main menu : Edit -> Project Configuration. I believe you can and it can be really useful too.
"You can't add new meshes to an existing project".