How to fork a file (git cp ?)

I need to add a new thing to our project. The thing will be similar to what already exists. I'd like to "derive" the new files from the existing ones -- without altering them and by preserving the change-history.

This is not a separate branch -- the "forked" files will co-exist. I'd call this git-cp (analogous to git-mv), but it does not exist...

Any ideas? Thanks!

    -mi