In theory, someone could write a script that processes bitcoind's $DATADIR/blocks directory into bootstrap.dat, but no one has written that yet.
They have now...
https://gist.github.com/dooglus/5002764It uses the Armory code to do all the work, and creates a bootstrap.dat with the correct sha256 hash:
$ sha256sum bootstrap.dat
bf658c7055b733bfc15ea167f298c5599b89d220b14dbe7c8ef20b18e468c451 bootstrap.dat