I do not think Rave Reports has any inbuilt function which converts numbers to works.  However, I have found a article, which explains How to Convert Numbers (Currency) to Words, on About.com.
Have a look at the ConvertToWords function in the NumberToWords unit.
You could convert the number and pass the words to the report at runtime.
Edit:
You can pass parameters to your reports like this.
  RvProject.Open;
  RvProject.SelectReport('ParametrizedReport',False);
  RvProject.SetParam('Name','Leonel');
  RvProject.Execute;
  RvProject.Close;
Have a look at the Introduction to Rave Reports article.