I've never heard of a way to get spaceships into a 3d model format without first going through the creature editor and using the colladaexport command. The tool that the OP used to open a spaceship in the creature editor is
a hack and is only available for Windows at the moment.
While it seems unlikely that opening the creation's PNG file in a 3d modeling program will allow you to access the 3d model of the creation (if it did, then why would Maxis bother to add the colladaexport command?), you may as well try that and see if it works.
one way I know of to convert a .dae to a .obj file. Not sure if there's others.