Hi Alan,
I don’t think you can change that in your Preferences; ENTER is sort of a ‘special key’, and confirm is not explicitly mapped to any key. If it were possible, you’d have to go to everything that uses ENTER to confirm and Add a copy, that uses Spacebar to confirm. That would not be worth the trouble (and again, i don’t think it is even possible) in my opinion.
If you come from Rhino, I’d suggest to get used to Blender’s Keymap and use that.
If you switch between the two, that may or may not be ideal.
You might be able to remap your Spacebar to do what ENTER does in your OS and hopefully be able to turn that behavior on, only when using Blender. It’ll depend on your OS, how easy that is to do (if at all possible).
