You can set up a git commit hook, so that when you commit to git, it will append "admin" to the commit message. Then when you do your git svn dcommit, your commit messages will already have the expected string there.
Documentation on git hooks.
In your project's .git directory, there will be a directory called hooks.
cwd: ~/testrepo/.git/hooks master
λ > ls
applypatch-msg.sample post-update.sample pre-commit.sample prepare-commit-msg.sample
commit-msg.sample pre-applypatch.sample pre-rebase.sample update.sample
You can take a look at the files, prepare-commit-message.sample can be used to edit commit messages before they are committed.
Make a copy of prepare-commit-message.sample and call it prepare-commit-message.
cp prepare-commit-message.sample prepare-commit-message
So open that file, and as a demonstration I added this line to the end:
# Append 'admin' to the end of the commit message, $1 is the message passed as argument
echo "admin" >> "$1"
Save changes, exit, try a change, and commit it.
λ > echo etc >> README
λ > git add .
λ > git commit -m "testing"
[master 89a435d] testing admin
1 files changed, 1 insertions(+), 0 deletions(-)
λ > git log
commit 89a435d5e110229d3c9989bfb464ae2420eb5088
Author: birryree
Date: Fri Oct 28 12:54:20 2011 -0400
testing
admin