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If you would like to make it as solid, when you import the file, you can pick 'Auto sew geometry', and the file will be imported to be Solid.
If the imported file is not solid, simply make it as solid by using SEW function from Heal tab in ZW3D. If it is opened in other program, I believe that there are similar SEW function there.
In order for better collaboration, we will find better way to enhance the import and export, thanks for your detailed info.