can cruise control be added on after factory?

Yeah, it can.  But my suggestion would be that you take it to the dealer for that.  I have worked on cars for a while now, and when OEM cruise parts are installed, it will work, look, and operate much better than an aftermarket system you are going to get when not going to a dealer.

Hope this helps:)

Second that advice.  Definitely use OEM parts.

if you want to do it yourself ford offers a complete kit pn 9371-4763-aa...hope this helps

i just put it the kit larry was talking about.. its a very easy install took about a hour, to a hour and a half.. and i don't know shit about cars...

good luck friend

I just got off the phone with the guys at fordspartscenter.net regarding part number 9371-4763-aa and was told that that kit was only for 06-07, nothing for the 08 Focus.  

David7812,  You said you installed it and got it to work on your car..  is it an 08 as well?  Did you have any problems installing it or with the funcions?  

Thanks!