I think you have two options here:
- You can create a ship from any blueprint you already have opening it and going to "File>Save as". This will create a new file to save the ship apart from the original one.
- You can create a blueprint from some premade ships going to "File>Open premade blueprint", then you can save it to another file and work on it.
However, if you mean you have an assembled ship and you want to automatically convert that ship into an editable blueprint, that is not possible.
Have a great day!