The easiest way with Sweet Home 3D would be to:
- import the model 3D file in an empty home
- export it to OBJ format with "3D view > Export to OBJ format..."
- edit the MTL file generated along with OBJ file with a text editor
- change the transparency line that starts with "d xxx" to "d 1"
- save MTL file
- import the exported OBJ file