From experience, I've been able to UV unwrap a model in 3dcoat then export it as a .obj and import to maya, add the extra mesh (while making sure everything is still combined, merged and clean up is good), go into Maya's UV texture editor and scale down the new UVs for the new areas of geometry in an an area with free space. Re-import into 3d-coat, then re-unwrap the seams on the new geometry, while still keeping your old unwrap (providing you applied UV set and so forth before the export).
This may differ depending on the 3d package you use. Also, make sure to not have too many UV shells (especially overlapping ones) when adding the new geometry, as this may crash 3dcoat unless you apply a new automatic unwrap.