Adding History

Is there a way to add history to a repository?
We have just started using git, where before we had nothing but backup
directories all over the place.  We created the git repository using our
existing source directory.  Is there a way to now go and add some of the backup
source directories to the history?  Or would this break all the SHA1 of the
current objects?  I am not sure it would be worth it at this point.  But we are
still pretty early in our use of git so now would be the easiest time.  

Roger