Is it possible to compress a tar ball with gzip/bzip2/xz after tar ball file has been created?

If we create a tar ball file by giving the following command tar -cvf Docs.tar $HOME/Documents/* then post creation of the tar ball is it possible to use gzip or bzip2 or xz or some other compression utility to compress the tar file? I know that we can give the option --bzip2 or --xz or --gzip while creating the tar along with -cvf option but what if that is not done. And after the tar is created then the compression is sought to be applied. Is it possible? If yes then how?