Visual Studio installer projects have a completely different format than the C++ project .vcxproj files. As far as I know the details of their format are not documented.
I would take Visual Studio's inability to recognize and export these deployment projects as a sign that you would be better served by adding new installer projects to your solutions instead of attempting to copy existing ones from other solutions. Of course, you could copy the folder containing the installer project .vdproj file from one solution to another and make whatever edits you believe are needed. I have no idea what those edits should be.