What kind of existing project are you referring to ?
A seam-gen style project ?
Then there is not much to it besides enabling Seam on it under Project properties. Unfortuanetly since seam-gen projects is not really geared towards eclipse dev so somethings won't work as easy as with seam wizard generated projects.
You can see the release announcement for how to setup archiving and just remember that Seam artifact wizards will only work with Seam wizard projects, but code completion etc. will work by just enabling seam.