How to append a new paragraph into text of a shape?

here is my program
tell application “Omnigraffle Professional 5”
set text of OmnigraphID to text of OmnigraphID & “new words as a new line” --this ID is sent by omnigraffle to indicated a shape.
– I hope the new line in text is at paragraph 4 while there are already have 3 paragraphs.
end tell

     the result is that all style are changed. unless I do as below
   set text of paragraph 4 of text of OmnigraphID to "new words as a new line"
   set size of ...
   set color of ...

My question is :
How can I add a new paragraph after text of OmnigraphID ?

thanks anybody for answer me!