How to add inner liner(shell) to outer shell?

I have a helmet model that was created for me for pepakura. When I import the obj file to any 3d program I just have the outer shell and a hole at the neck. So it is difficult to do a 3d print. I would like to add liner or a inner shell to the inside the helmet so that I can 3d print the helmet. If someone can help or point me in the right direction that would be very helpful. If I can use a free 3d program (blender) to make these fixes the better. I am a beginner/intermediate level. I did work on CAD stuff back in high school but that was more than 15 years ago and software has changed haha.