Is there a way to update email body in email routing worker?

From the docs I see that

 interface EmailMessage<Body = unknown> {
  readonly from: string;
  readonly to: string;
  readonly headers: Headers;
  readonly raw: ReadableStream;
  readonly rawSize: number;

  setReject(reason: String): void;
  forward(rcptTo: string, headers?: Headers): Promise<void>;

ReadableStream is readonly. and ReadableStream has only

pipeTo(destination, options)

I am a bit lost.
is the possibility to do it in an email worker?

I don’t see any mentions of modifying the message body in the docs. I’d assume this isn’t supported and the properties being readonly confirms that. For sending email from Workers you could use MailChannels.