Hi,
It is possible through the repository API, assuming you have a recent version of Review Board installed:
The repository API is pretty specialized, with different arguments depending on what type of repository you're installing, but basically, you'd need to issue a HTTP POST to /api/repositories with the following fields (using multi-part/form-data as the POST body format):
* name (The name of the repository)
* path (The main path to the repository, which Review Board will communicate with)
* tool (You will set this to "Subversion")
* username (The username used to access the repository, if needed)
* password (The password used to access the repository, if needed)
You can probably leave everything else blank, but it depends on your needs.
If SSH access is used to talk to the repository, make sure you have a SSH key already configured in Review Board and that each repository can be accessed via that key.
Christian