1) Create a new project from the Novel template.
2) Place the cursor in some section text and choose Format > Style > Create New Style from Selection. Name the new style something like "Chapter Subtitle."
3) Apply the desired subtitle formatting (e.g. center alignment) to the text.
4) Choose Format > Style > Redefine Style from Selection to update the Chapter Subtitle style to include this formatting.
5) Optionally, editing the Chapter Title style and set the "next" style to be "Chapter Subtitle."
I've attached a sample project (set in Times New Roman) that illustrates these steps.