Frontmatter can be manipulated by passing a FrontmatterProcessor, which makes changes to the frontmatter object, in options.ts.

export type Options = {
  // other options
  frontmatterProcessor?: FrontmatterProcessor;
};

export type FrontmatterProcessor = (
  cwd: string,
  file: string,
  baseRoute: string,
  frontmatter: UnknownFrontMatter
) => void;