Have repository {start-repo} and want to merge into {old-repo} at {some-dir-name}
# if you just wanted trunk replace with this
# svndump {start-repo} | svndumpfilter include trunk
svnadmin dump {start-repo} > my-dump
# copy file to main server and change to web-user
svnadmin load --parent-dir {some-dir-name} {old-repo} < my-dump