Hi rwazejewski,
post_8 is right, you can do it that way.
You can add a new mesh to your original scene in your 3dapp, reexport them all and replace the mesh in your painter project. Very important thing : you have to keep the same texture sets number and names for your original mesh; if not, it won't work.
Other thing, sometimes i work on a character in one scene and a prop in another. when i want to merge the 2 in one scene, for exemple the prop in the character scene, i convert all my layer stack of my prop into a smart material, merge the 2 meshes in one scene like said before, and re-apply on my prop the smart material.
You won't keep all the brushes operations you did on your added mesh, but at least you have it textured.