[Users] Insert a Header into ACK message

[fancy]

Dear All:
I've tried many methods to add Max-Forwarsd header into ACK message (message 8), all methods I tried were fail.
According to RFC 3261 8.1.1.6, Max-Forwards must exist in this ACK message. Can anyone tell me how to add a header into ACK message? Thank you very much.

     UA11     :   NUT(Proxy 1) :       UA12
      |       :        |       :        |
      |-------:------->|       :        | 1. INVITE 
      |<------:--------|       :        | 2. 407  
      |-------:------->|       :        | 3. ACK 
      |       :        |       :        | 
      |-------:------->|       :        | 4. INVITE 
      |       :        |-------:------->| 5. INVITE 
      |<------:--------|       :        | 6. 100  
      |       :        |       :        |
      |       :        |<------:--------| 7. 486 (Busy Here)
      |       :        |       :        |
      |       :        |-------:------->| 8. ACK
      |       :        |       :        |
      |<------:--------|       :        | 9. 486
      |       :        |       :        |
      |-------:------->|       :        |10. ACK
      |       :        |       :        |

Best Regards,
Fangyu Ling