[Edit: The original reply below is from pre SVN 1.7, and there this was the best way to solve the problem (although it is not the primary use case of svnsync). In SVN 1.7 client or later there is the svnrdump tool that does more directly what you are trying to achieve)]
use svnsync to synchronize the source to the destination (needs admin access to the destination repository or at least a way to add hooks, but no special access to the source repository). If the destination already has revisions, sync the source to a temporary repository, and then use svn-merge-repos.pl to merge the two local repositories.