By that I ment you can't open up current ship blueprints from modern versions in smedit ( you would need to use an older version like your doing), you can only import with the newer versions.
I also recently came across these two tools:
Which still work and make life much easier, allowing you to change entity type and core placement on the fly!
How I do the file conversion:
(there is also a program called Vox2Sm3 which turns in straight into a blueprint, but I havn't used it a whole lot, it just means you don't need to use SMEdit)
Download binvox: (you can get one version of it here)
Programs and editors/Binvox
You can find SMEdit here:
Place your .obj file (can use Sketckup to export models as obj) in the same directory as binvox.exe
Open command promt and navigate to the folder with binvox.
Fro example, this is my one:
Then type in binvox followed by whatever commands you want then the file name.
E.g and then press enter. A binvox file will appear in that directory which you can then open in SMEdit
Then you just open SMEdit and import, do what you want then export as a blueprint :P (e.g further rotation, core, adding wedges to smooth, changing colour)