I do this by having a "razor submissions" folder as well as my normal junk folder.
spamassassin-tagged spam goes into Junk.
other mail I identify as spam goes into Junk as well (via the Junk mail button in Mail.app)
Every now and then I review the contents of Junk, take out any false matches, and drag the rest into my "razor submissions folder".
Then I have a small script that runs every 2 minutes from cron, taking each message from "razor submissions" and reporting it to spamassassin (and razor, dcc etc)
Works fairly well.
Darrin
#!/bin/sh
cd $HOME/"Maildir/.Razor Submissions"
# Quick exit if lockfile exists [ -e .submitting.lock ] && exit 0
lockfile -l60 .submitting.lock
cd new for i in *; do [ -e $i ] && mv $i ../cur/ done
cd ..
for i in cur/*; do [ -e $i ] && /usr/bin/spamassassin -r < $i && rm $i done
rm -f .submitting.lock