You might want to provide more details on what you are specifically
trying to accomplish - are you looking to automate this process to
be run regularly, or is it just a one-time task? Are there a lot of
documents to be copied?

If it is just a one-time thing, I'd say the easiest way to do this
is not through code, but to go to your document  library, switch to
Explorer View, right-click and copy  the files  from that document
library you want to bring into the new one, and paste them into the
new one (also using Explorer View). Then again you might have a lot
of documents in the document library  and might not want to rummage
through the entire library, but I don't know for sure.

Please note that copying  through Explorer view screws up the versioning if
there is any in the doclib. Trust me, it looks like it works but when you
did into it, everything is FUBAR.

Sometimes solutions that seem too good to
be true are just that. We haven't really dabbled with
document versioning much here since our go-live but this will be
good to keep in mind if/once we get to that point.