Is there a simple way to copy a directory from one repository into another repository with copying all of the history?
The simplest way is using:
svnadmin dump path/to/repos > repos.out
This will create a portable format for your repository (with history) in the file repos.out. You can then use
svnadmin load path/to/newrepos < repos.out
to load your 'dumped' repository to the new or existing one.
Chapter 5. Repository Maintenance -> Migrating Repository Data Elsewhere has this note about using svnadmin dump as of version 1.7:
The Subversion repository dump format describes versioned repository changes only. It will not carry any information about uncommitted transactions, user locks on filesystem paths, repository or server configuration customizations (including hook scripts), and so on.