Once you have the tag in local repository you can do something like
git merge tags/yourtag
If you don’t have the “trunk” tags locally, you can fetch it using
git fetch remote-url "refs/tags/*:refs/tags/*"
Or by setting up the remote
git remote add upstream remote-url
and fetching the stuff using
git fetch -t upstream
I think, though, using
git remote update
will have similar effect.