php message system with system message [on hold]

I have a messaging system in my site that work fine.

the table look like this:

msg_id
from_id
to_id
sender_ip
subject
content
time
status

But now I need to send a user message form our site system (domain name).

I tried to make a user with special id, but it looks bad.

How can I make this work ?

What needs to be in the {from_id} table, because "from_id" is related to the user table, but if I want to send message from the site system (its not a user).