So in order to play as novorussiya, you would select the outcome form civilisation and make ukraine form novorussiya.
This means that Ukraine will completely turn into novorussiya however we can fix this with another outcome for annexation and choose Ukraine again to annex all the lands you want Ukraine to have but leave the land you want novorussiya to have
Hopefully when this fires you will play as novorussiya and Ukraine will still be present. I believe their may be an issue in that Ukrainian territories may have a novorussiyan population, but that will not effect you too much hopefully
In the event to stay as Ukraine you will just need to select the outcome annexation for novorussiya and select the territories you want them to have