sieve forwarding with extra text

[Ramprasad]

Hi,
  We have a cyrus installation( on linux redhat 9)   with around 110k
mailboxes. I want to allow users to setup forwarding to different email
ids ( probably more than 1 ) 

Can I setup sieve so that the email forwarded will have special text
attached to it ? 

Also I am not sure how sieve would scale for 110k mailboxes , will it be
too heavy a load ? 

Thanks
Ram